feat: split kiosk into fallback canvas and OSD overlay layers (#8)

Drive two independent visual layers per display target from one shared
video presence signal: the fullscreen fallback/menu canvas below video
and a transparent, input-transparent OSD overlay above video, reserved
as the future flight OSD rendering surface. WFB and video decode stay
out of the UI process; pilot video keeps rendering priority.

README.md

@@ -16,9 +16,10 @@ Current capabilities:
 - logs channel changes instead of every frame;
 - starts/stops `fpv-video` from the configured CH6 power/lifecycle control;
 - starts an independent kiosk display before video, tolerates temporary X11 downtime without restarting the supervisor, and stops deterministically on shutdown;
-- keeps the fallback canvas below `fpv-video`, with `Brak połączenia WFB` while video is absent.
+- keeps the fullscreen fallback/menu canvas below `fpv-video`, showing `Brak połączenia WFB` only while video is absent;
+- maps a transparent, input-transparent OSD overlay above `fpv-video` as the reserved flight OSD rendering surface: it never presents pixels over the video or intercepts its input.
 
-Future MIPI/touch targets use their own template rather than sharing one global layout. Flight OSD will use a separate transparent layer above video, preserving the pilot stream's priority.
+Future MIPI/touch targets use their own templates rather than sharing one global layout. The flight OSD will render on the existing transparent layer above video, preserving the pilot stream's priority.
 
 ## Installation on Orange Pi
 
@@ -57,5 +58,5 @@ serial:
 ## Runtime dependencies
 
 - X11 session on `:0` owned by `orangepi`; the kiosk waits for X11 and reconnects after display-manager restarts;
-- `wmctrl` for fullscreen/below kiosk state; without it the canvas maps as a normal window and the condition is logged;
+- `wmctrl` for the fullscreen/stacking kiosk window states; without it the kiosk windows map as normal windows and the condition is logged;
 - `runuser` from util-linux for the unprivileged kiosk launch.

cmd/groundstation-display/main.go

@@ -1,4 +1,4 @@
-// Command groundstation-display renders the Ground Station kiosk canvas. It
+// Command groundstation-display renders the Ground Station kiosk layers. It
 // owns its display lifecycle independently of WFB, the transmitter and the
 // video launcher: it waits for its target to appear, redraws as needed, and
 // stops only when it receives SIGINT or SIGTERM.
@@ -16,19 +16,31 @@ import (
 	"git.piksar.eu/piotr/groundstation/internal/display"
 )
 
-// buildTarget selects the target-specific opener and template. Each target
-// owns its own backend and layouts; there is no shared global template.
-// buildTarget wires one shared video presence signal into both the opener
-// (which observes the video output lifecycle) and the fallback template
-// (which shows or hides itself from it).
-func buildTarget(name string) (display.Opener, display.Template, error) {
+// buildTarget selects the target-specific layers. Each target owns its own
+// backend and layouts; there is no shared global template. The Viture target
+// builds two independent layers — the fullscreen fallback/menu canvas below
+// video and the transparent OSD overlay above it — wired to one shared video
+// presence signal: the sessions observe the video output lifecycle on the
+// display server and the templates draw or hide from it. WFB and video
+// decode never move into this process.
+func buildTarget(name string) ([]display.Layer, error) {
 	switch name {
 	case "x11":
 		video := display.NewVideoPresence()
-		return display.X11Opener{Video: video},
-			display.NewFallbackTemplate(video, "Brak połączenia WFB"), nil
+		return []display.Layer{
+			{
+				Name:     "fallback-canvas",
+				Opener:   display.X11Opener{Video: video},
+				Template: display.NewFallbackTemplate(video, "Brak połączenia WFB"),
+			},
+			{
+				Name:     "osd-overlay",
+				Opener:   display.X11Opener{Video: video, Role: display.RoleAbove},
+				Template: display.NewOSDTemplate(video),
+			},
+		}, nil
 	default:
-		return nil, nil, fmt.Errorf("unknown display target %q (supported: x11)", name)
+		return nil, fmt.Errorf("unknown display target %q (supported: x11)", name)
 	}
 }
 
@@ -36,7 +48,7 @@ func main() {
 	target := flag.String("target", "x11", "display target: x11 (Viture) today; mipi planned")
 	flag.Parse()
 
-	opener, template, err := buildTarget(*target)
+	layers, err := buildTarget(*target)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -44,9 +56,7 @@ func main() {
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer stop()
 
-	// Temporary X11 unavailability never exits this process; display.Run
+	// Temporary X11 unavailability never exits this process; every layer
 	// keeps retrying internally until the context is cancelled.
-	if err := display.Run(ctx, opener, template, display.Options{}); err != nil {
-		log.Fatalf("display: %v", err)
-	}
+	display.RunLayers(ctx, layers)
 }

cmd/groundstation-display/main_test.go

@@ -19,45 +19,67 @@ func (c *recordingCanvas) Text(_, _ int, message string) {
 }
 func (c *recordingCanvas) Flush() { c.flushes++ }
 
-func TestBuildTargetWiresSharedVideoPresenceToFallback(t *testing.T) {
-	opener, template, err := buildTarget("x11")
+func TestBuildTargetBuildsIndependentVitureLayers(t *testing.T) {
+	layers, err := buildTarget("x11")
 	if err != nil {
 		t.Fatal(err)
 	}
+	if len(layers) != 2 {
+		t.Fatalf("layers=%d want the fallback canvas and the OSD overlay", len(layers))
+	}
+	canvas, osd := layers[0], layers[1]
+	if canvas.Name != "fallback-canvas" || osd.Name != "osd-overlay" {
+		t.Fatalf("layer names %q/%q want fallback-canvas/osd-overlay", canvas.Name, osd.Name)
+	}
 
-	x11, ok := opener.(display.X11Opener)
+	canvasOpener, ok := canvas.Opener.(display.X11Opener)
 	if !ok {
-		t.Fatalf("opener %T must be the X11 target", opener)
+		t.Fatalf("canvas opener %T must be the X11 target", canvas.Opener)
 	}
-	if x11.Video == nil {
-		t.Fatal("x11 opener must observe the video presence signal")
+	if canvasOpener.Role != display.RoleBelow {
+		t.Fatalf("canvas layer role %v must stay below video", canvasOpener.Role)
+	}
+	osdOpener, ok := osd.Opener.(display.X11Opener)
+	if !ok {
+		t.Fatalf("OSD opener %T must be the X11 target", osd.Opener)
+	}
+	if osdOpener.Role != display.RoleAbove {
+		t.Fatalf("OSD layer role %v must stay above video", osdOpener.Role)
 	}
 
-	canvas := &recordingCanvas{width: 800, height: 600}
-	template.Draw(canvas)
-	if len(canvas.lines) != 1 || canvas.lines[0] != "Brak połączenia WFB" {
-		t.Fatalf("lines=%q want the exact fallback text", canvas.lines)
+	// Both layers must share one presence signal so the video lifecycle
+	// observed by either session drives both templates.
+	if canvasOpener.Video == nil || osdOpener.Video == nil || canvasOpener.Video != osdOpener.Video {
+		t.Fatal("layers must share one video presence signal")
 	}
 
-	// The opener and the template must share one presence signal so the
-	// video lifecycle observed by the session drives the fallback.
-	x11.Video.SetActive(true)
-	canvas.lines = nil
-	template.Draw(canvas)
-	if len(canvas.lines) != 0 {
-		t.Fatalf("lines=%q must disappear while video is present", canvas.lines)
+	// The canvas layer keeps the target-specific fallback template with the
+	// exact fallback text.
+	canvasSurface := &recordingCanvas{width: 800, height: 600}
+	canvas.Template.Draw(canvasSurface)
+	if len(canvasSurface.lines) != 1 || canvasSurface.lines[0] != "Brak połączenia WFB" {
+		t.Fatalf("canvas lines=%q want the exact fallback text", canvasSurface.lines)
 	}
 
-	x11.Video.SetActive(false)
-	canvas.lines = nil
-	template.Draw(canvas)
-	if len(canvas.lines) != 1 || canvas.lines[0] != "Brak połączenia WFB" {
-		t.Fatalf("lines=%q want the fallback back after video end", canvas.lines)
+	// The OSD layer is the OSD surface, never the fallback.
+	osdSurface := &recordingCanvas{width: 800, height: 600}
+	osd.Template.Draw(osdSurface)
+	if len(osdSurface.lines) != 0 {
+		t.Fatalf("OSD lines=%q want none: the overlay must not duplicate the fallback", osdSurface.lines)
 	}
+
+	// Video presence hides the fallback on the canvas layer only.
+	canvasOpener.Video.SetActive(true)
+	canvasSurface.lines = nil
+	canvas.Template.Draw(canvasSurface)
+	if len(canvasSurface.lines) != 0 {
+		t.Fatalf("canvas lines=%q must disappear while video is present", canvasSurface.lines)
+	}
+	canvasOpener.Video.SetActive(false)
 }
 
 func TestBuildTargetRejectsUnknownTarget(t *testing.T) {
-	if _, _, err := buildTarget("hdmi-mirror"); err == nil {
+	if _, err := buildTarget("hdmi-mirror"); err == nil {
 		t.Fatal("unknown target must fail")
 	}
 }

internal/display/display.go

@@ -1,17 +1,21 @@
-// Package display owns the Ground Station kiosk canvas lifecycle. The kiosk
-// runs independently of WFB, the transmitter and the RC video knob: it waits
-// for its rendering target to appear, keeps a fullscreen black canvas alive
-// and stops only when told to. While decoded pilot video is absent the
-// canvas shows the fallback; when video output is presented the video window
-// takes rendering priority and the fallback disappears, returning when video
-// ends. Presence is observed from the video output itself on the display
-// server, never from WFB or launcher processes. Each target (X11/Viture
-// today, MIPI/touch later) provides its own opener and templates instead of
-// one global layout.
+// Package display owns the Ground Station kiosk layers. The kiosk runs
+// independently of WFB, the transmitter and the RC video knob: it waits for
+// its rendering target to appear, keeps its windows alive and stops only
+// when told to. Each target drives two independent visual layers: a
+// fullscreen fallback/menu canvas below video, visible while decoded pilot
+// video is absent, and a transparent, input-transparent OSD overlay above
+// video, reserved as the future flight OSD rendering surface. When video
+// output is presented the video window takes rendering priority and the
+// fallback disappears, returning when video ends. Presence is observed from
+// the video output itself on the display server, never from WFB or launcher
+// processes, and the video pipeline is never moved into, coupled to or
+// touched by this package. Each target (X11/Viture today, MIPI/touch later)
+// provides its own openers and templates instead of one global layout.
 package display
 
 import (
 	"context"
+	"sync"
 	"time"
 )
 
@@ -108,6 +112,39 @@ func Run(ctx context.Context, opener Opener, template Template, options Options)
 	}
 }
 
+// Layer is one independent visual layer of a display target: its own
+// connection, window role and template. A lost or degraded layer never
+// affects the others, so a missing overlay cannot disturb the fallback
+// canvas and neither can disturb pilot video.
+type Layer struct {
+	// Name identifies the layer in diagnostics.
+	Name string
+	// Opener connects to the layer's own window on the target.
+	Opener Opener
+	// Template renders the layer's target-specific layout.
+	Template Template
+	// Options tune the layer's lifecycle loop. Zero values select the
+	// defaults.
+	Options Options
+}
+
+// RunLayers drives every layer of a display target until ctx is cancelled,
+// each on its own connection with its own reconnect backoff. A layer whose
+// target keeps failing keeps retrying and never stops the other layers, so
+// the Ground Station supervisor stays up. RunLayers returns only after every
+// layer has stopped.
+func RunLayers(ctx context.Context, layers []Layer) {
+	var wg sync.WaitGroup
+	for _, layer := range layers {
+		wg.Add(1)
+		go func(layer Layer) {
+			defer wg.Done()
+			_ = Run(ctx, layer.Opener, layer.Template, layer.Options)
+		}(layer)
+	}
+	wg.Wait()
+}
+
 func nextDelay(delay, max time.Duration) time.Duration {
 	delay *= 2
 	if delay > max {

internal/display/display_test.go

@@ -242,6 +242,87 @@ func TestRunReconnectsAfterSessionFailure(t *testing.T) {
 	}
 }
 
+func TestRunLayersDrawsEveryLayerOnItsOwnCanvas(t *testing.T) {
+	canvasSession := newFakeSession()
+	osdSession := newFakeSession()
+	layers := []Layer{
+		{
+			Name:     "fallback-canvas",
+			Opener:   fakeOpener{open: func(int) (Session, error) { return canvasSession, nil }},
+			Template: NewStatusTemplate("canvas"),
+		},
+		{
+			Name:     "osd-overlay",
+			Opener:   fakeOpener{open: func(int) (Session, error) { return osdSession, nil }},
+			Template: NewOSDTemplate(NewVideoPresence()),
+		},
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() {
+		RunLayers(ctx, layers)
+		close(done)
+	}()
+
+	waitFor(t, func() bool { return canvasSession.canvas.lineCount() == 1 }, "canvas layer draw")
+	waitFor(t, func() bool {
+		clears, _ := osdSession.canvas.drawCounts()
+		return clears == 1
+	}, "osd layer transparent clear")
+	cancel()
+
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("run layers did not stop after cancel")
+	}
+	if got := canvasSession.closeCount(); got != 1 {
+		t.Fatalf("canvas session closed %d times, want 1", got)
+	}
+	if got := osdSession.closeCount(); got != 1 {
+		t.Fatalf("osd session closed %d times, want 1", got)
+	}
+}
+
+func TestRunLayersKeepsOtherLayerAliveWhenOneTargetFails(t *testing.T) {
+	alive := newFakeSession()
+	layers := []Layer{
+		{
+			Name:     "dead",
+			Opener:   fakeOpener{open: func(int) (Session, error) { return nil, errors.New("target down") }},
+			Template: NewStatusTemplate("never drawn"),
+		},
+		{
+			Name:     "alive",
+			Opener:   fakeOpener{open: func(int) (Session, error) { return alive, nil }},
+			Template: NewOSDTemplate(NewVideoPresence()),
+		},
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() {
+		RunLayers(ctx, layers)
+		close(done)
+	}()
+
+	waitFor(t, func() bool {
+		clears, _ := alive.canvas.drawCounts()
+		return clears == 1
+	}, "alive layer keeps drawing while the other target fails")
+	cancel()
+
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("run layers did not stop after cancel")
+	}
+	if got := alive.closeCount(); got != 1 {
+		t.Fatalf("alive session closed %d times, want 1", got)
+	}
+}
+
 func TestRunStopsWithoutConnectingWhenContextAlreadyCancelled(t *testing.T) {
 	var opens int32
 	opener := fakeOpener{open: func(int) (Session, error) {

internal/display/osd.go

@@ -0,0 +1,23 @@
+package display
+
+// OSDTemplate is the flight OSD layout of a display target: a transparent,
+// input-transparent surface above pilot video, reserved as the future OSD
+// rendering surface. It presents nothing yet and never draws the fallback:
+// the OSD is a telemetry surface, not a video substitute, and WFB or video
+// decode never move into the UI process to feed it. Flight telemetry
+// rendering arrives in later Ground Station work; the shared presence keeps
+// the OSD on the same video lifecycle as the fallback canvas, so the session
+// redraws it on every presence transition.
+type OSDTemplate struct {
+	video *VideoPresence
+}
+
+// NewOSDTemplate builds the OSD surface for a target.
+func NewOSDTemplate(video *VideoPresence) OSDTemplate {
+	return OSDTemplate{video: video}
+}
+
+func (t OSDTemplate) Draw(canvas Canvas) {
+	canvas.Clear()
+	canvas.Flush()
+}

internal/display/osd_test.go

@@ -0,0 +1,43 @@
+package display
+
+import "testing"
+
+func TestOSDTemplateStaysTransparentWhileVideoAbsent(t *testing.T) {
+	canvas := &fakeCanvas{width: 800, height: 600}
+
+	NewOSDTemplate(NewVideoPresence()).Draw(canvas)
+
+	clears, flushes := canvas.drawCounts()
+	if clears != 1 || flushes != 1 {
+		t.Fatalf("clears=%d flushes=%d want 1 and 1", clears, flushes)
+	}
+	if lines := canvas.lines(); len(lines) != 0 {
+		t.Fatalf("OSD lines=%q want none: the OSD is a telemetry surface, not a fallback", lines)
+	}
+}
+
+func TestOSDTemplateStaysTransparentWhileVideoPresent(t *testing.T) {
+	canvas := &fakeCanvas{width: 800, height: 600}
+	presence := NewVideoPresence()
+	presence.SetActive(true)
+
+	NewOSDTemplate(presence).Draw(canvas)
+
+	if lines := canvas.lines(); len(lines) != 0 {
+		t.Fatalf("OSD lines=%q want none while video is present", lines)
+	}
+}
+
+func TestOSDTemplateNeverDrawsTheFallbackText(t *testing.T) {
+	canvas := &fakeCanvas{width: 800, height: 600}
+	presence := NewVideoPresence()
+	template := NewOSDTemplate(presence)
+
+	for _, active := range []bool{false, true, false} {
+		presence.SetActive(active)
+		template.Draw(canvas)
+		if lines := canvas.lines(); len(lines) != 0 {
+			t.Fatalf("lines=%q after presence=%v want none", lines, active)
+		}
+	}
+}

internal/display/x11.go

@@ -11,6 +11,7 @@ import (
 	"unicode/utf16"
 
 	"github.com/BurntSushi/xgb"
+	"github.com/BurntSushi/xgb/shape"
 	"github.com/BurntSushi/xgb/xproto"
 )
 
@@ -26,8 +27,93 @@ const (
 	maxImageText16Units = 127
 	// windowTypePropertyName is the EWMH property naming a window's role.
 	windowTypePropertyName = "_NET_WM_WINDOW_TYPE"
+	// layerMarkerPropertyName tags Ground Station layer windows so sibling
+	// layers — and their window-manager frames — never count as video
+	// output for each other.
+	layerMarkerPropertyName = "_GROUNDSTATION_LAYER"
 )
 
+// osdEventMask selects only repaint bookkeeping for the OSD overlay: no
+// pointer, key or motion events, so the server never routes device input to
+// the overlay even before its empty input shape is applied.
+const osdEventMask = xproto.EventMaskExposure
+
+// deviceInputEventMask are the pointer and key selection bits a layer above
+// video must never select: device input belongs to the windows below.
+const deviceInputEventMask = xproto.EventMaskKeyPress | xproto.EventMaskKeyRelease |
+	xproto.EventMaskButtonPress | xproto.EventMaskButtonRelease |
+	xproto.EventMaskPointerMotion
+
+// LayerRole orders a Ground Station window against presented video output.
+// The fallback/menu canvas stays below video; the OSD overlay stays above.
+type LayerRole int
+
+const (
+	// RoleBelow marks the fullscreen fallback/menu canvas under video.
+	RoleBelow LayerRole = iota
+	// RoleAbove marks the transparent, input-transparent OSD overlay above
+	// video.
+	RoleAbove
+)
+
+// layerMarkerValue is the layer marker payload identifying a role's window
+// as our own.
+func layerMarkerValue(role LayerRole) string {
+	if role == RoleAbove {
+		return "osd"
+	}
+	return "kiosk"
+}
+
+// markerIsGroundstation reports whether a layer marker payload marks one of
+// our own windows: those are Ground Station layers, never video output.
+func markerIsGroundstation(marker string) bool {
+	return marker == layerMarkerValue(RoleBelow) || marker == layerMarkerValue(RoleAbove)
+}
+
+// ewmhWindowTypeName is the EWMH window role announced for a layer. The OSD
+// overlay announces DOCK, the always-above helper role, so window managers
+// never decorate or bury it; the fallback canvas keeps announcing nothing,
+// as before.
+func ewmhWindowTypeName(role LayerRole) string {
+	if role == RoleAbove {
+		return "_NET_WM_WINDOW_TYPE_DOCK"
+	}
+	return ""
+}
+
+// fullscreenState is the wmctrl stacking state for a layer role.
+func fullscreenState(role LayerRole) string {
+	if role == RoleAbove {
+		return "above"
+	}
+	return "below"
+}
+
+// stackModeForVideoAppear is where a layer goes relative to the first
+// presented video window: the canvas parks below it, the OSD rises above it.
+func stackModeForVideoAppear(role LayerRole) uint32 {
+	if role == RoleAbove {
+		return xproto.StackModeAbove
+	}
+	return xproto.StackModeBelow
+}
+
+// raisesOnVideoEnd reports whether a layer raises itself when video ends:
+// only the below canvas needs that to make the fallback visible again; the
+// OSD overlay is already above everything.
+func raisesOnVideoEnd(role LayerRole) bool {
+	return role == RoleBelow
+}
+
+// transparentShapeKinds are the shape regions cleared to empty so the OSD
+// overlay is transparent and passes input through: an empty input shape
+// never receives pointer or key events, and an empty bounding shape presents
+// nothing over the video below — with or without a compositor.
+func transparentShapeKinds() []shape.Kind {
+	return []shape.Kind{shape.SkInput, shape.SkBounding}
+}
+
 // ewmhNonVideoTypes are EWMH auxiliary window roles that never count as
 // video output: desktop shells, panels, menus, dialogs and notifications
 // must not suppress the fallback. Windows without the property — typical
@@ -48,22 +134,27 @@ var ewmhNonVideoTypes = map[string]bool{
 }
 
 // X11Opener connects to an X server (selected by DISPLAY in the environment)
-// and creates the fullscreen kiosk canvas. It is the display target for the
-// Viture screen; future targets such as MIPI/touch provide their own Opener
-// and templates.
+// and maps one fullscreen kiosk window for the layer role it is built with:
+// the fallback/menu canvas below video (RoleBelow) or the transparent,
+// input-transparent OSD overlay above video (RoleAbove). It is the display
+// target for the Viture screen; future targets such as MIPI/touch provide
+// their own Opener and templates.
 type X11Opener struct {
 	// PollInterval is how often X events are drained while waiting, so that
 	// cancellation stays responsive. Zero selects 100 ms.
 	PollInterval time.Duration
-	// Fullscreen applies the fullscreen/below window state. It defaults to
+	// Fullscreen applies the fullscreen window state. It defaults to
 	// shelling out to wmctrl; tests may replace it. A failure is logged and
-	// the canvas degrades to a plain window.
+	// the window degrades to a plain window.
 	Fullscreen func(args []string) error
 	// Video is the shared decoded-video presence signal. The session feeds
 	// it from the video output window's own lifecycle on the root screen;
-	// templates consume it to show or hide the fallback. When nil the
-	// opener tracks presence in a private signal.
+	// templates consume it to show or hide themselves. When nil the opener
+	// tracks presence in a private signal.
 	Video *VideoPresence
+	// Role orders this layer's window against presented video: RoleBelow
+	// for the fallback/menu canvas, RoleAbove for the OSD overlay.
+	Role LayerRole
 }
 
 func (o X11Opener) presence() *VideoPresence {
@@ -102,11 +193,46 @@ func (o X11Opener) Open(ctx context.Context) (Session, error) {
 		return nil, fmt.Errorf("allocate window: %w", err)
 	}
 
+	// The OSD overlay selects no device input even before its empty input
+	// shape exists; the canvas keeps its legacy exposure-only mask.
+	eventMask := uint32(xproto.EventMaskExposure)
+	if o.Role == RoleAbove {
+		eventMask = osdEventMask
+	}
 	values := uint32(xproto.CwBackPixel | xproto.CwEventMask)
 	xproto.CreateWindow(conn, screen.RootDepth, window, screen.Root,
 		0, 0, screen.WidthInPixels, screen.HeightInPixels, 0,
 		xproto.WindowClassInputOutput, screen.RootVisual, values,
-		[]uint32{screen.BlackPixel, xproto.EventMaskExposure})
+		[]uint32{screen.BlackPixel, eventMask})
+	// Tag the window as a Ground Station layer and announce its EWMH role
+	// before mapping, so no observer or window manager ever sees an
+	// untagged frame of our own layers.
+	markerAtom := internAtom(conn, layerMarkerPropertyName)
+	if markerAtom != 0 {
+		marker := layerMarkerValue(o.Role)
+		xproto.ChangeProperty(conn, xproto.PropModeReplace, window, markerAtom,
+			xproto.AtomString, 8, uint32(len(marker)), []byte(marker))
+	}
+	if name := ewmhWindowTypeName(o.Role); name != "" {
+		typeProp := internAtom(conn, windowTypePropertyName)
+		roleAtom := internAtom(conn, name)
+		if typeProp != 0 && roleAtom != 0 {
+			value := make([]byte, 4)
+			binary.LittleEndian.PutUint32(value, uint32(roleAtom))
+			xproto.ChangeProperty(conn, xproto.PropModeReplace, window, typeProp,
+				xproto.AtomAtom, 32, 1, value)
+		}
+	}
+	// The OSD overlay must be transparent and input-transparent from its
+	// first mapped frame: the shapes are emptied before mapping, and a
+	// server without the shape extension never gets the overlay at all, so
+	// it can never present an unshaped frame over pilot video.
+	if o.Role == RoleAbove {
+		if err := applyTransparentShapes(conn, window); err != nil {
+			conn.Close()
+			return nil, err
+		}
+	}
 	xproto.MapWindow(conn, window)
 	// Watch the root screen so video output announces itself: a video
 	// window mapping is the first presented video frame, its unmap or
@@ -119,26 +245,27 @@ func (o X11Opener) Open(ctx context.Context) (Session, error) {
 		return nil, fmt.Errorf("map kiosk window: %w", err)
 	}
 
-	// A kiosk restart must never cover running video: if video windows are
-	// already presented, record presence and hand rendering priority to the
-	// topmost one before the fallback is drawn anywhere.
+	// A layer reconnect must never cover running video: if video windows
+	// are already presented, record presence and take our side relative to
+	// the topmost one before anything is drawn.
 	presence := o.presence()
 	typeAtom := internWindowTypeAtom(conn)
-	videoWindows := scanVideoWindows(conn, screen.Root, window, typeAtom)
+	videoWindows := scanVideoWindows(conn, screen.Root, window, typeAtom, markerAtom)
 	if len(videoWindows) > 0 {
 		presence.SetActive(true)
-		stackWindowBelow(conn, window, videoWindows[len(videoWindows)-1])
+		stackWindowRelative(conn, window, videoWindows[len(videoWindows)-1],
+			stackModeForVideoAppear(o.Role))
 	} else {
 		presence.SetActive(false)
 	}
 
-	// Fullscreen on the root screen, and kept below the video window: pilot
-	// video owns rendering priority whenever it is available. The kiosk
-	// raises itself only after video ends, to bring the fallback back; a
-	// missing wmctrl only degrades to a plain window and must not kill the
-	// kiosk.
-	if err := o.applyFullscreen(x11FullscreenArgs(uint32(window))); err != nil {
-		log.Printf("kiosk fullscreen state: %v (canvas stays a plain window)", err)
+	// Fullscreen on the root screen, on our side of the video window: pilot
+	// video owns rendering priority whenever it is available. The canvas
+	// raises itself only after video ends, to bring the fallback back; the
+	// OSD overlay is already above video. A missing wmctrl only degrades to
+	// a plain window and must not kill the layer.
+	if err := o.applyFullscreen(x11FullscreenArgs(uint32(window), o.Role)); err != nil {
+		log.Printf("kiosk fullscreen state: %v (window stays a plain window)", err)
 	}
 
 	gc, err := xproto.NewGcontextId(conn)
@@ -171,6 +298,8 @@ func (o X11Opener) Open(ctx context.Context) (Session, error) {
 		video:        presence,
 		videoWindows: videoWindowSet(videoWindows),
 		typeAtom:     typeAtom,
+		markerAtom:   markerAtom,
+		role:         o.Role,
 		unicodeFont:  unicodeFont,
 	}, nil
 }
@@ -183,53 +312,152 @@ func videoWindowSet(windows []xproto.Window) map[xproto.Window]struct{} {
 	return set
 }
 
-func x11FullscreenArgs(window uint32) []string {
-	return []string{"-i", "-r", fmt.Sprintf("0x%08x", window), "-b", "add,fullscreen,below"}
+func x11FullscreenArgs(window uint32, role LayerRole) []string {
+	return []string{"-i", "-r", fmt.Sprintf("0x%08x", window),
+		"-b", "add,fullscreen," + fullscreenState(role)}
+}
+
+// applyTransparentShapes empties the overlay's input and bounding shapes
+// before it is mapped: it then never intercepts pointer or key input and
+// never presents pixels over pilot video, with or without a compositor.
+// Future OSD content will punch its output shape per drawn region.
+func applyTransparentShapes(conn *xgb.Conn, window xproto.Window) error {
+	if err := shape.Init(conn); err != nil {
+		return fmt.Errorf("shape extension: %w", err)
+	}
+	for _, kind := range transparentShapeKinds() {
+		if err := shape.RectanglesChecked(conn, shape.SoSet, kind, 0,
+			window, 0, 0, nil).Check(); err != nil {
+			return fmt.Errorf("empty shape region %d: %w", kind, err)
+		}
+	}
+	return nil
 }
 
 // scanVideoWindows lists already-presented video windows on the root screen
-// in bottom-to-top stacking order, so a kiosk reconnect can yield priority
-// to the topmost one and never cover running video.
-func scanVideoWindows(conn *xgb.Conn, root, own xproto.Window, typeAtom xproto.Atom) []xproto.Window {
+// in bottom-to-top stacking order, so a layer reconnect can take its side
+// relative to the topmost one and never cover running video.
+func scanVideoWindows(conn *xgb.Conn, root, own xproto.Window, typeAtom, markerAtom xproto.Atom) []xproto.Window {
 	tree, err := xproto.QueryTree(conn, root).Reply()
 	if err != nil {
 		return nil
 	}
 	var windows []xproto.Window
 	for _, w := range tree.Children {
-		if w != own && presentedVideoWindow(conn, w, typeAtom) {
+		if w != own && presentedVideoWindow(conn, w, typeAtom, markerAtom) {
 			windows = append(windows, w)
 		}
 	}
 	return windows
 }
 
-// presentedVideoWindow reports whether w is a visible output window. Only a
-// real, presented window counts as video: input-only and override-redirect
-// helpers such as menus or notifications never do, and neither do windows
-// carrying an EWMH auxiliary window type.
-func presentedVideoWindow(conn *xgb.Conn, w xproto.Window, typeAtom xproto.Atom) bool {
+// windowFacts is the presentation evidence gathered for one candidate
+// window, so the video decision stays a pure function.
+type windowFacts struct {
+	inputOutput      bool
+	overrideRedirect bool
+	viewable         bool
+	auxiliary        bool
+	groundstation    bool
+}
+
+// isPresentedVideoWindow decides whether a window is video output. Only a
+// real, presented output window counts as video: input-only and
+// override-redirect helpers such as menus or notifications never do, and
+// neither do windows carrying an EWMH auxiliary window type or a Ground
+// Station layer marker.
+func isPresentedVideoWindow(f windowFacts) bool {
+	return f.inputOutput && !f.overrideRedirect && f.viewable &&
+		!f.auxiliary && !f.groundstation
+}
+
+// presentedVideoWindow gathers the presentation facts of w.
+func presentedVideoWindow(conn *xgb.Conn, w xproto.Window, typeAtom, markerAtom xproto.Atom) bool {
 	attrs, err := xproto.GetWindowAttributes(conn, w).Reply()
 	if err != nil {
 		return false
 	}
-	return attrs.Class == xproto.WindowClassInputOutput &&
-		!attrs.OverrideRedirect &&
-		attrs.MapState == xproto.MapStateViewable &&
-		!auxiliaryWindowType(conn, w, typeAtom)
+	return isPresentedVideoWindow(windowFacts{
+		inputOutput:      attrs.Class == xproto.WindowClassInputOutput,
+		overrideRedirect: attrs.OverrideRedirect,
+		viewable:         attrs.MapState == xproto.MapStateViewable,
+		auxiliary:        auxiliaryWindowType(conn, w, typeAtom),
+		groundstation:    groundstationLayerWindow(conn, w, markerAtom),
+	})
 }
 
-// internWindowTypeAtom resolves the EWMH window type property atom, or 0
-// when the server refuses, disabling the auxiliary type filter.
-func internWindowTypeAtom(conn *xgb.Conn) xproto.Atom {
+// markerWalkBounds bound the descendant search for layer markers: window
+// managers reparent client windows into frame windows, so a Ground Station
+// layer must be recognized through its frame, and the search must stay
+// cheap even on a busy desktop screen. depth must cover our window below
+// one window-manager frame; nodes bounds the total windows inspected.
+func markerWalkBounds() (depth, nodes int) {
+	return 4, 64
+}
+
+// groundstationLayerWindow reports whether w itself or any window below it
+// carries our layer marker. Window managers frame our windows, so the
+// marker must be found through frame windows too: otherwise a framed
+// Ground Station layer could count as video output and permanently
+// suppress the fallback. When the search hits its bounds or the server
+// refuses, it reports not-found, degrading to the plain window checks.
+func groundstationLayerWindow(conn *xgb.Conn, w xproto.Window, markerAtom xproto.Atom) bool {
+	if markerAtom == 0 {
+		return false
+	}
+	depth, nodes := markerWalkBounds()
+	visited := 0
+	var walk func(xproto.Window, int) bool
+	walk = func(w xproto.Window, depth int) bool {
+		if visited >= nodes {
+			return false
+		}
+		visited++
+		if carriesLayerMarker(conn, w, markerAtom) {
+			return true
+		}
+		if depth <= 0 {
+			return false
+		}
+		tree, err := xproto.QueryTree(conn, w).Reply()
+		if err != nil {
+			return false
+		}
+		for _, child := range tree.Children {
+			if walk(child, depth-1) {
+				return true
+			}
+		}
+		return false
+	}
+	return walk(w, depth)
+}
+
+// carriesLayerMarker reports whether w itself carries our layer marker.
+func carriesLayerMarker(conn *xgb.Conn, w xproto.Window, markerAtom xproto.Atom) bool {
+	reply, err := xproto.GetProperty(conn, false, w, markerAtom,
+		xproto.GetPropertyTypeAny, 0, 64).Reply()
+	if err != nil || reply.ValueLen == 0 {
+		return false
+	}
+	return markerIsGroundstation(string(reply.Value))
+}
+
+// internAtom resolves an atom by name, or 0 when the server refuses,
+// disabling the checks that would use it.
+func internAtom(conn *xgb.Conn, name string) xproto.Atom {
 	reply, err := xproto.InternAtom(conn, false,
-		uint16(len(windowTypePropertyName)), windowTypePropertyName).Reply()
+		uint16(len(name)), name).Reply()
 	if err != nil {
 		return 0
 	}
 	return reply.Atom
 }
 
+func internWindowTypeAtom(conn *xgb.Conn) xproto.Atom {
+	return internAtom(conn, windowTypePropertyName)
+}
+
 // auxiliaryWindowType reports whether the window carries an EWMH window
 // type naming a non-video helper role. A missing property means the window
 // counts as video.
@@ -255,10 +483,11 @@ func auxiliaryWindowType(conn *xgb.Conn, w xproto.Window, typeAtom xproto.Atom)
 	return false
 }
 
-func stackWindowBelow(conn *xgb.Conn, window, sibling xproto.Window) {
+// stackWindowRelative moves window to the given side of sibling.
+func stackWindowRelative(conn *xgb.Conn, window, sibling xproto.Window, mode uint32) {
 	xproto.ConfigureWindow(conn, window,
 		xproto.ConfigWindowSibling|xproto.ConfigWindowStackMode,
-		[]uint32{uint32(sibling), xproto.StackModeBelow})
+		[]uint32{uint32(sibling), mode})
 }
 
 // videoChange is the video-lifecycle meaning of one root structure event.
@@ -328,6 +557,8 @@ type x11Session struct {
 	video        *VideoPresence
 	videoWindows map[xproto.Window]struct{}
 	typeAtom     xproto.Atom
+	markerAtom   xproto.Atom
+	role         LayerRole
 	unicodeFont  bool
 }
 
@@ -395,34 +626,39 @@ func (s *x11Session) WaitEvent(ctx context.Context) (bool, error) {
 }
 
 // applyVideoChange folds one root structure event into the shared video
-// presence and restacks the kiosk so video keeps rendering priority. It
-// reports whether the template must be redrawn: video appearing makes the
-// fallback disappear and video ending brings it back. Presence follows the
-// set of presented video windows, so an unrelated window appearing or
-// ending can never hide or resurrect the fallback on its own.
+// presence and restacks this layer so video keeps rendering priority: the
+// canvas parks below video and the OSD overlay rises above it. It reports
+// whether the template must be redrawn: video appearing makes the fallback
+// disappear and video ending brings it back. Presence follows the set of
+// presented video windows, so an unrelated window appearing or ending can
+// never hide or resurrect a template on its own.
 func (s *x11Session) applyVideoChange(event xgb.Event) bool {
 	change, w := classifyVideoChange(event, s.window)
 	switch change {
 	case videoAppeared:
-		// Menus, notifications and other auxiliary windows never count as
-		// video; only a presented output window does.
-		if !presentedVideoWindow(s.conn, w, s.typeAtom) {
+		// Menus, notifications, other auxiliary windows and our own sibling
+		// layers never count as video; only a presented output window does.
+		if !presentedVideoWindow(s.conn, w, s.typeAtom, s.markerAtom) {
 			return false
 		}
 		if !addVideoWindow(s.videoWindows, w) {
 			return false
 		}
-		// The first presented video output takes priority: park below it.
-		stackWindowBelow(s.conn, s.window, w)
+		// The first presented video output takes priority: park on our side
+		// of it.
+		stackWindowRelative(s.conn, s.window, w, stackModeForVideoAppear(s.role))
 		return s.video.SetActive(true)
 	case videoDisappeared:
 		if !removeVideoWindow(s.videoWindows, w) {
 			return false
 		}
-		// Raise the kiosk so the fallback returns visibly even if some
-		// other window was stacked above it.
-		xproto.ConfigureWindow(s.conn, s.window,
-			xproto.ConfigWindowStackMode, []uint32{xproto.StackModeAbove})
+		// Only the below canvas raises itself so the fallback returns
+		// visibly even if some other window was stacked above it; the OSD
+		// overlay is already above video.
+		if raisesOnVideoEnd(s.role) {
+			xproto.ConfigureWindow(s.conn, s.window,
+				xproto.ConfigWindowStackMode, []uint32{xproto.StackModeAbove})
+		}
 		return s.video.SetActive(false)
 	default:
 		return false

internal/display/x11_test.go

@@ -5,18 +5,25 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/BurntSushi/xgb/shape"
 	"github.com/BurntSushi/xgb/xproto"
 )
 
-func TestX11FullscreenArgsKeepCanvasBelowVideo(t *testing.T) {
-	args := x11FullscreenArgs(0x1a2b)
-
-	joined := strings.Join(args, " ")
-	if !strings.Contains(joined, "add,fullscreen,below") {
-		t.Fatalf("args %v must request fullscreen with below stacking", args)
+func TestX11FullscreenArgsOrderByRole(t *testing.T) {
+	below := strings.Join(x11FullscreenArgs(0x1a2b, RoleBelow), " ")
+	if !strings.Contains(below, "add,fullscreen,below") {
+		t.Fatalf("args %q must request fullscreen with below stacking", below)
 	}
-	if !strings.Contains(joined, "0x00001a2b") {
-		t.Fatalf("args %v must reference the kiosk window id", args)
+	if !strings.Contains(below, "0x00001a2b") {
+		t.Fatalf("args %q must reference the kiosk window id", below)
+	}
+
+	above := strings.Join(x11FullscreenArgs(0x1a2b, RoleAbove), " ")
+	if strings.Contains(above, "below") {
+		t.Fatalf("args %q must never stack the OSD overlay below video", above)
+	}
+	if !strings.Contains(above, "add,fullscreen,above") {
+		t.Fatalf("args %q must request fullscreen with above stacking", above)
 	}
 }
 
@@ -36,7 +43,7 @@ func TestX11OpenerAppliesFullscreenArgsThroughHook(t *testing.T) {
 		return errors.New("wmctrl missing")
 	}}
 
-	if err := opener.applyFullscreen(x11FullscreenArgs(0x42)); err == nil {
+	if err := opener.applyFullscreen(x11FullscreenArgs(0x42, RoleBelow)); err == nil {
 		t.Fatal("hook error must propagate for logging")
 	}
 	if len(got) != 1 || !strings.Contains(strings.Join(got[0], " "), "add,fullscreen,below") {
@@ -189,3 +196,104 @@ func TestX11FontPatternRequestsUnicodeFixedFont(t *testing.T) {
 		t.Fatalf("font pattern %q must select a Unicode encoding for the exact fallback text", iso10646FontPattern)
 	}
 }
+
+func TestLayerMarkerIdentifiesOwnLayerWindows(t *testing.T) {
+	if layerMarkerValue(RoleBelow) == layerMarkerValue(RoleAbove) {
+		t.Fatal("roles must carry distinct marker payloads")
+	}
+	for _, role := range []LayerRole{RoleBelow, RoleAbove} {
+		if !markerIsGroundstation(layerMarkerValue(role)) {
+			t.Fatalf("marker %q must identify our own window", layerMarkerValue(role))
+		}
+	}
+	for _, foreign := range []string{"", "video", "desktop", "groundstation-fake"} {
+		if markerIsGroundstation(foreign) {
+			t.Fatalf("foreign marker payload %q must not identify our windows", foreign)
+		}
+	}
+}
+
+func TestMarkerWalkBoundsStayBoundedAndCoverFrameDepth(t *testing.T) {
+	depth, nodes := markerWalkBounds()
+	if depth <= 0 || nodes <= 0 {
+		t.Fatalf("marker walk bounds depth=%d nodes=%d must be positive", depth, nodes)
+	}
+	if nodes < depth {
+		t.Fatalf("node budget %d must cover at least a depth-%d ancestor chain", nodes, depth)
+	}
+	// A reparenting window manager puts each of our two layer windows inside
+	// its own frame, so the walk must reach our window below one frame.
+	if depth < 2 {
+		t.Fatalf("marker walk depth %d must cover our window below a window-manager frame", depth)
+	}
+}
+
+func TestEWMHWindowTypeAnnouncesOverlayRoleOnly(t *testing.T) {
+	if name := ewmhWindowTypeName(RoleAbove); !strings.Contains(name, "DOCK") {
+		t.Fatalf("OSD overlay window type %q must be the always-above helper role", name)
+	}
+	if name := ewmhWindowTypeName(RoleBelow); name != "" {
+		t.Fatalf("fallback canvas must keep announcing no window type, got %q", name)
+	}
+}
+
+func TestPresentedVideoDecisionExcludesOwnLayersAndHelpers(t *testing.T) {
+	video := windowFacts{inputOutput: true, viewable: true}
+	if !isPresentedVideoWindow(video) {
+		t.Fatal("a presented output window must count as video")
+	}
+	for name, facts := range map[string]windowFacts{
+		"input-only":           {viewable: true},
+		"override-redirect":    {inputOutput: true, viewable: true, overrideRedirect: true},
+		"unmapped":             {inputOutput: true},
+		"auxiliary":            {inputOutput: true, viewable: true, auxiliary: true},
+		"groundstation-canvas": {inputOutput: true, viewable: true, groundstation: true},
+		"groundstation-osd":    {inputOutput: true, viewable: true, groundstation: true},
+	} {
+		if isPresentedVideoWindow(facts) {
+			t.Fatalf("%s window must never count as video", name)
+		}
+	}
+	// Window-manager frames around our layer windows carry no marker of
+	// their own; groundstationLayerWindow finds the marker below the frame,
+	// so framed Ground Station layers land in these same excluded facts.
+}
+
+func TestStackOrderByRoleKeepsLayersOnTheirSideOfVideo(t *testing.T) {
+	if mode := stackModeForVideoAppear(RoleBelow); mode != xproto.StackModeBelow {
+		t.Fatalf("canvas must park below appearing video, got %v", mode)
+	}
+	if mode := stackModeForVideoAppear(RoleAbove); mode != xproto.StackModeAbove {
+		t.Fatalf("OSD overlay must rise above appearing video, got %v", mode)
+	}
+	if !raisesOnVideoEnd(RoleBelow) {
+		t.Fatal("canvas must raise itself when video ends so the fallback returns")
+	}
+	if raisesOnVideoEnd(RoleAbove) {
+		t.Fatal("OSD overlay is already above video and must not restack on video end")
+	}
+}
+
+func TestOSDEventMaskNeverSelectsDeviceInput(t *testing.T) {
+	if osdEventMask&deviceInputEventMask != 0 {
+		t.Fatalf("OSD event mask must not select pointer or key input: %b", osdEventMask)
+	}
+	if osdEventMask&xproto.EventMaskExposure == 0 {
+		t.Fatal("OSD event mask must still select exposure for redraws")
+	}
+}
+
+func TestTransparentShapesEmptyInputAndOutput(t *testing.T) {
+	var input, output bool
+	for _, kind := range transparentShapeKinds() {
+		switch kind {
+		case shape.SkInput:
+			input = true
+		case shape.SkBounding:
+			output = true
+		}
+	}
+	if !input || !output {
+		t.Fatalf("shape kinds %v must empty both the input and the bounding region", transparentShapeKinds())
+	}
+}