From 6563f1bbebc732b1db30922f904dcfabbf9a4f29 Mon Sep 17 00:00:00 2001
From: Philipp Hasper <vcs@hasper.info>
Date: Wed, 3 Jun 2026 17:10:40 +0200
Subject: [PATCH] Fix system gesture triggering when editing quad corners.

On devices without a back button (e.g. Pixel10), dragging corners when
close to the screen borders triggered the system back gesture.

Adding exclusion areas for each of the corner handles fixes this.
It was not possible to exclude the whole image area, as Android seems to
silently limit the area of allowed exclusion.

The mandatory system gestures like the home or quick-switch gesture cannot
be excluded via areas. The only solution here is to introduce a bottom
padding which moves the image view upwards, out of the gesture area.

Both changes are no-ops for devices without system gestures, so no change
there.

Closes #193
---
 .../app/ui/screens/crop/CropScreen.kt         |  6 ++++
 .../app/ui/screens/crop/QuadOverlay.kt        | 28 ++++++++++++++++++-
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/app/src/main/java/org/fairscan/app/ui/screens/crop/CropScreen.kt b/app/src/main/java/org/fairscan/app/ui/screens/crop/CropScreen.kt
index 1818abb..cc907b5 100644
--- a/app/src/main/java/org/fairscan/app/ui/screens/crop/CropScreen.kt
+++ b/app/src/main/java/org/fairscan/app/ui/screens/crop/CropScreen.kt
@@ -27,6 +27,7 @@ import androidx.compose.foundation.gestures.waitForUpOrCancellation
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.WindowInsets
 import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.mandatorySystemGestures
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.safeDrawing
 import androidx.compose.foundation.layout.windowInsetsPadding
@@ -81,6 +82,10 @@ fun CropScreen(
     BackHandler { navigation.back() }
 
     val isLandscape = isLandscape(LocalConfiguration.current)
+    val density = LocalDensity.current
+    val bottomInsetForSystemGestures = with(density) {
+        WindowInsets.mandatorySystemGestures.getBottom(density).toDp()
+    }
 
     MyScaffold(
         navigation = navigation,
@@ -102,6 +107,7 @@ fun CropScreen(
                         contentDescription = "Image to edit",
                         modifier = Modifier
                             .fillMaxSize()
+                            .padding(bottom = bottomInsetForSystemGestures)
                             .onGloballyPositioned { coordinates ->
                                 state.containerSize = coordinates.size
                             },
diff --git a/app/src/main/java/org/fairscan/app/ui/screens/crop/QuadOverlay.kt b/app/src/main/java/org/fairscan/app/ui/screens/crop/QuadOverlay.kt
index d5fd193..437e5af 100644
--- a/app/src/main/java/org/fairscan/app/ui/screens/crop/QuadOverlay.kt
+++ b/app/src/main/java/org/fairscan/app/ui/screens/crop/QuadOverlay.kt
@@ -16,14 +16,18 @@ package org.fairscan.app.ui.screens.crop
 
 import androidx.compose.foundation.Canvas
 import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.systemGestureExclusion
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.geometry.Rect
 import androidx.compose.ui.unit.IntSize
 import org.fairscan.imageprocessing.Point
 import org.fairscan.imageprocessing.Quad
 import org.fairscan.imageprocessing.scaledTo
+import kotlin.math.max
+import kotlin.math.min
 
 @Composable
 fun QuadOverlay(
@@ -35,7 +39,15 @@ fun QuadOverlay(
     val quadColor = MaterialTheme.colorScheme.primary
     val handleColor = quadColor.copy(alpha = 0.5f)
 
-    Canvas(modifier = modifier.fillMaxSize()) {
+    Canvas(modifier = modifier
+        .fillMaxSize()
+        // Android limits the exclusion areas, so we cannot simply exclude the whole quad/image.
+        // Instead, the corners must get excluded individually.
+        .systemGestureExclusion { cornerExclusionRect(quad.topLeft, containerSize, displaySize) }
+        .systemGestureExclusion { cornerExclusionRect(quad.topRight, containerSize, displaySize) }
+        .systemGestureExclusion { cornerExclusionRect(quad.bottomRight, containerSize, displaySize) }
+        .systemGestureExclusion { cornerExclusionRect(quad.bottomLeft, containerSize, displaySize) }
+    ) {
         val scaledQuad = quad.scaledTo(
             fromWidth = 1,
             fromHeight = 1,
@@ -74,3 +86,17 @@ fun QuadOverlay(
 }
 
 fun Point.toOffset() = Offset(x.toFloat(), y.toFloat())
+
+private fun cornerExclusionRect(
+    corner: Point,
+    containerSize: IntSize,
+    displaySize: IntSize
+): Rect {
+    val pos = QuadCoordinateUtils.normalizedToScreen(corner, containerSize, displaySize)
+    val r = QuadEditingHandler.CORNER_TOUCH_RADIUS
+    return Rect(max(.0f, pos.x - r),
+                max(.0f, pos.y - r),
+                min(containerSize.width.toFloat(), pos.x + r),
+                min(containerSize.height.toFloat(), pos.y + r)
+    )
+}