fix a rare crash around blurring in newer versions of flutter - bit of a workaround

lib/widgets/add_node_sheet.dart

@@ -594,10 +594,13 @@ class _AddNodeSheetState extends State<AddNodeSheet> {
             ],
             ),
             
-            // Tutorial overlay - show only if tutorial should be shown and we're done checking
+            // Tutorial overlay - always mounted once the initial check
-            if (!_isCheckingTutorial && _showTutorial)
+            // completes (never conditionally inserted/removed - see
+            // PositioningTutorialOverlay's class doc for why), fades via
+            // its own `visible` flag instead.
+            if (!_isCheckingTutorial)
               Positioned.fill(
-                child: PositioningTutorialOverlay(),
+                child: PositioningTutorialOverlay(visible: _showTutorial),
               ),
           ],
         );

lib/widgets/edit_node_sheet.dart

@@ -627,10 +627,13 @@ class _EditNodeSheetState extends State<EditNodeSheet> {
             ],
             ),
 
-            // Tutorial overlay - show only if tutorial should be shown and we're done checking
+            // Tutorial overlay - always mounted once the initial check
-            if (!_isCheckingTutorial && _showTutorial)
+            // completes (never conditionally inserted/removed - see
+            // PositioningTutorialOverlay's class doc for why), fades via
+            // its own `visible` flag instead.
+            if (!_isCheckingTutorial)
               Positioned.fill(
-                child: PositioningTutorialOverlay(),
+                child: PositioningTutorialOverlay(visible: _showTutorial),
               ),
           ],
         );

lib/widgets/positioning_tutorial_overlay.dart

@@ -6,23 +6,95 @@ import '../services/localization_service.dart';
 
 /// Overlay that appears over add/edit node sheets to guide users through
 /// the positioning tutorial. Shows a blurred background with tutorial text.
-class PositioningTutorialOverlay extends StatelessWidget {
+///
+/// Stability note: on Android's Impeller (Vulkan) renderer, BackdropFilter
+/// snapshots the render pass's "back texture" mid-frame. If that snapshot
+/// races against the surface being invalidated - which happens constantly
+/// right where this overlay lives, since the enclosing bottom sheet resizes
+/// on open/keyboard/rotation - Impeller can hit a null texture and hard-abort
+/// the whole process natively (not a catchable Dart error). Two things keep
+/// that race from being triggered here:
+///  - [RepaintBoundary] isolates the filter onto its own stable composited
+///    layer instead of sharing one with the actively-resizing sheet.
+///  - The widget is always kept mounted by the parent sheet and toggles
+///    visibility via a fade (see [visible]) rather than being abruptly
+///    inserted/removed from the tree, which is believed to be the other half
+///    of what triggers the crash.
+class PositioningTutorialOverlay extends StatefulWidget {
   const PositioningTutorialOverlay({
     super.key,
+    required this.visible,
     this.onFadeOutComplete,
   });
 
-  /// Called when the fade-out animation completes (if animated)
+  /// Whether the tutorial should currently be showing. Toggling this fades
+  /// the overlay in/out - see class doc for why it's never removed from
+  /// the tree instead.
+  final bool visible;
+
+  /// Called when the fade-out animation completes (i.e. the overlay has
+  /// become fully transparent after [visible] turned false).
   final VoidCallback? onFadeOutComplete;
 
+  @override
+  State<PositioningTutorialOverlay> createState() => _PositioningTutorialOverlayState();
+}
+
+class _PositioningTutorialOverlayState extends State<PositioningTutorialOverlay>
+    with SingleTickerProviderStateMixin {
+  late final AnimationController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = AnimationController(
+      duration: const Duration(milliseconds: 300),
+      vsync: this,
+      value: widget.visible ? 1.0 : 0.0,
+    );
+    _controller.addStatusListener(_onStatusChanged);
+  }
+
+  void _onStatusChanged(AnimationStatus status) {
+    if (status == AnimationStatus.dismissed) {
+      widget.onFadeOutComplete?.call();
+    }
+  }
+
+  @override
+  void didUpdateWidget(covariant PositioningTutorialOverlay oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (widget.visible != oldWidget.visible) {
+      if (widget.visible) {
+        _controller.forward();
+      } else {
+        _controller.reverse();
+      }
+    }
+  }
+
+  @override
+  void dispose() {
+    _controller.removeStatusListener(_onStatusChanged);
+    _controller.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
-    return AnimatedBuilder(
+    // Ignore touches when faded out so the invisible-but-still-mounted
+    // overlay doesn't block interaction with the sheet underneath.
+    return IgnorePointer(
+      ignoring: !widget.visible,
+      child: FadeTransition(
+        opacity: _controller,
+        child: AnimatedBuilder(
           animation: LocalizationService.instance,
           builder: (context, child) {
             final locService = LocalizationService.instance;
 
             return ClipRect(
+              child: RepaintBoundary(
                 child: BackdropFilter(
                   filter: ImageFilter.blur(
                     sigmaX: kPositioningTutorialBlurSigma,
@@ -85,8 +157,11 @@ class PositioningTutorialOverlay extends StatelessWidget {
                     ),
                   ),
                 ),
+              ),
             );
           },
+        ),
+      ),
     );
   }
 }