Workaround rounding erros in cupertino nav bar transition (#88935)

packages/flutter/lib/src/cupertino/nav_bar.dart

@@ -80,6 +80,52 @@ class _HeroTag {
   }
 }
 
+// An `AnimatedWidget` that imposes a fixed size on its child widget, and
+// shifts the child widget in the parent stack, driven by its `offsetAnimation`
+// property.
+class _FixedSizeSlidingTransition extends AnimatedWidget {
+  const _FixedSizeSlidingTransition({
+    Key? key,
+    required this.isLTR,
+    required this.offsetAnimation,
+    required this.size,
+    required this.child,
+  }) : super(key: key, listenable: offsetAnimation);
+
+  // Whether the writing direction used in the navigation bar transition is
+  // left-to-right.
+  final bool isLTR;
+
+  // The fixed size to impose on `child`.
+  final Size size;
+
+  // The animated offset from the top-leading corner of the stack.
+  //
+  // When `isLTR` is true, the `Offset` is the position of the child widget in
+  // the stack render box's regular coordinate space.
+  //
+  // When `isLTR` is false, the coordinate system is flipped around the
+  // horizontal axis and the origin is set to the top right corner of the render
+  // boxes. In other words, this parameter describes the offset from the top
+  // right corner of the stack, to the top right corner of the child widget, and
+  // the x-axis runs right to left.
+  final Animation<Offset> offsetAnimation;
+
+  final Widget child;
+
+  @override
+  Widget build(BuildContext context) {
+    return Positioned(
+      top: offsetAnimation.value.dy,
+      left: isLTR ? offsetAnimation.value.dx : null,
+      right: isLTR ? null : offsetAnimation.value.dx,
+      width: size.width,
+      height: size.height,
+      child: child,
+    );
+  }
+}
+
 /// Returns `child` wrapped with background and a bottom border if background color
 /// is opaque. Otherwise, also blur with [BackdropFilter].
 ///
@@ -1709,48 +1755,73 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  // Create a Tween that moves a widget between its original position in its
-  // ancestor navigation bar to another widget's position in that widget's
-  // navigation bar.
+  // Create an animated widget that moves the given child widget between its
+  // original position in its ancestor navigation bar to another widget's
+  // position in that widget's navigation bar.
   //
   // Anchor their positions based on the vertical middle of their respective
   // render boxes' leading edge.
   //
-  // Also produce RelativeRects with sizes that would preserve the constant
-  // BoxConstraints of the 'from' widget so that animating font sizes etc don't
-  // produce rounding error artifacts with a linearly resizing rect.
-  RelativeRectTween slideFromLeadingEdge({
+  // This method assumes there's no other transforms other than translations
+  // when converting a rect from the original navigation bar's coordinate space
+  // to the other navigation bar's coordinate space, to avoid performing
+  // floating point operations on the size of the child widget, so that the
+  // incoming constraints used for sizing the child widget will be exactly the
+  // same.
+  _FixedSizeSlidingTransition slideFromLeadingEdge({
     required GlobalKey fromKey,
     required RenderBox fromNavBarBox,
     required GlobalKey toKey,
     required RenderBox toNavBarBox,
+    required Widget child,
   }) {
-    final RelativeRect fromRect = positionInTransitionBox(fromKey, from: fromNavBarBox);
-
     final RenderBox fromBox = fromKey.currentContext!.findRenderObject()! as RenderBox;
     final RenderBox toBox = toKey.currentContext!.findRenderObject()! as RenderBox;
 
-    // We move a box with the size of the 'from' render object such that its
-    // upper left corner is at the upper left corner of the 'to' render object.
-    // With slight y axis adjustment for those render objects' height differences.
-    Rect toRect =
-        toBox.localToGlobal(
-          Offset.zero,
-          ancestor: toNavBarBox,
-        ).translate(
-          0.0,
-          - fromBox.size.height / 2 + toBox.size.height / 2,
-        ) & fromBox.size; // Keep the from render object's size.
+    final bool isLTR = forwardDirection > 0;
 
-    if (forwardDirection < 0) {
-      // If RTL, move the center right to the center right instead of matching
-      // the center lefts.
-      toRect = toRect.translate(- fromBox.size.width + toBox.size.width, 0.0);
-    }
+    // The animation moves the fromBox so its anchor (left-center or right-center
+    // depending on the writing direction) aligns with toBox's anchor.
+    final Offset fromAnchorLocal = Offset(
+      isLTR ? 0 : fromBox.size.width,
+      fromBox.size.height / 2,
+    );
+    final Offset toAnchorLocal = Offset(
+      isLTR ? 0 : toBox.size.width,
+      toBox.size.height / 2,
+    );
+    final Offset fromAnchorInFromBox = fromBox.localToGlobal(fromAnchorLocal, ancestor: fromNavBarBox);
+    final Offset toAnchorInToBox = toBox.localToGlobal(toAnchorLocal, ancestor: toNavBarBox);
+
+    // We can't get ahold of the render box of the stack (i.e., `transitionBox`)
+    // we place components on yet, but we know the stack needs to be top-leading
+    // aligned with both fromNavBarBox and toNavBarBox to make the transition
+    // look smooth. Also use the top-leading point as the origin for ease of
+    // calculation.
+
+    // The offset to move fromAnchor to toAnchor, in transitionBox's top-leading
+    // coordinates.
+    final Offset translation = isLTR
+      ? toAnchorInToBox - fromAnchorInFromBox
+      : Offset(toNavBarBox.size.width - toAnchorInToBox.dx, toAnchorInToBox.dy)
+      - Offset(fromNavBarBox.size.width - fromAnchorInFromBox.dx, fromAnchorInFromBox.dy);
 
-    return RelativeRectTween(
-        begin: fromRect,
-        end: RelativeRect.fromRect(toRect, transitionBox),
+    final RelativeRect fromBoxMargin = positionInTransitionBox(fromKey, from: fromNavBarBox);
+    final Offset fromOriginInTransitionBox = Offset(
+      isLTR ? fromBoxMargin.left : fromBoxMargin.right,
+      fromBoxMargin.top,
+    );
+
+    final Tween<Offset> anchorMovementInTransitionBox = Tween<Offset>(
+      begin: fromOriginInTransitionBox,
+      end: fromOriginInTransitionBox + translation,
+    );
+
+    return _FixedSizeSlidingTransition(
+      isLTR: isLTR,
+      offsetAnimation: animation.drive(anchorMovementInTransitionBox),
+      size: fromBox.size,
+      child: child,
     );
   }
 
@@ -1846,13 +1917,11 @@ class _NavigationBarComponentsTransition {
 
     if (bottomMiddle != null && topBackLabel != null) {
       // Move from current position to the top page's back label position.
-      return PositionedTransition(
-        rect: animation.drive(slideFromLeadingEdge(
+      return slideFromLeadingEdge(
         fromKey: bottomComponents.middleKey,
         fromNavBarBox: bottomNavBarBox,
         toKey: topComponents.backLabelKey,
         toNavBarBox: topNavBarBox,
-        )),
         child: FadeTransition(
           // A custom middle widget like a segmented control fades away faster.
           opacity: fadeOutBy(bottomHasUserMiddle ? 0.4 : 0.7),
@@ -1903,13 +1972,11 @@ class _NavigationBarComponentsTransition {
 
     if (bottomLargeTitle != null && topBackLabel != null) {
       // Move from current position to the top page's back label position.
-      return PositionedTransition(
-        rect: animation.drive(slideFromLeadingEdge(
+      return slideFromLeadingEdge(
         fromKey: bottomComponents.largeTitleKey,
         fromNavBarBox: bottomNavBarBox,
         toKey: topComponents.backLabelKey,
         toNavBarBox: topNavBarBox,
-        )),
         child: FadeTransition(
           opacity: fadeOutBy(0.6),
           child: Align(
@@ -2063,13 +2130,11 @@ class _NavigationBarComponentsTransition {
     if (bottomLargeTitle != null &&
         topBackLabel != null &&
         bottomLargeExpanded) {
-      return PositionedTransition(
-        rect: animation.drive(slideFromLeadingEdge(
+      return slideFromLeadingEdge(
         fromKey: bottomComponents.largeTitleKey,
         fromNavBarBox: bottomNavBarBox,
         toKey: topComponents.backLabelKey,
         toNavBarBox: topNavBarBox,
-        )),
         child: FadeTransition(
           opacity: midClickOpacity ?? fadeInFrom(0.4),
           child: DefaultTextStyleTransition(
@@ -2088,13 +2153,11 @@ class _NavigationBarComponentsTransition {
     // The topBackLabel always comes from the large title first if available
     // and expanded instead of middle.
     if (bottomMiddle != null && topBackLabel != null) {
-      return PositionedTransition(
-        rect: animation.drive(slideFromLeadingEdge(
+      return slideFromLeadingEdge(
         fromKey: bottomComponents.middleKey,
         fromNavBarBox: bottomNavBarBox,
         toKey: topComponents.backLabelKey,
         toNavBarBox: topNavBarBox,
-        )),
         child: FadeTransition(
           opacity: midClickOpacity ?? fadeInFrom(0.3),
           child: DefaultTextStyleTransition(
@@ -2125,20 +2188,32 @@ class _NavigationBarComponentsTransition {
     }
 
     final RelativeRect to = positionInTransitionBox(topComponents.middleKey, from: topNavBarBox);
+    final RenderBox toBox = topComponents.middleKey.currentContext!.findRenderObject()! as RenderBox;
+
+    final bool isLTR = forwardDirection > 0;
+
+    // Anchor is the top-leading point of toBox, in transition box's top-leading
+    // coordinate space.
+    final Offset toAnchorInTransitionBox = Offset(
+      isLTR ? to.left : to.right,
+      to.top,
+    );
 
     // Shift in from the trailing edge of the screen.
-    final RelativeRectTween positionTween = RelativeRectTween(
-      begin: to.shift(
-        Offset(
-          forwardDirection * topNavBarBox.size.width / 2.0,
-          0.0,
-        ),
+    final Tween<Offset> anchorMovementInTransitionBox = Tween<Offset>(
+      begin: Offset(
+        // the "width / 2" here makes the middle widget's horizontal center on
+        // the trailing edge of the top nav bar.
+        topNavBarBox.size.width - toBox.size.width / 2,
+        to.top,
       ),
-      end: to,
+      end: toAnchorInTransitionBox,
     );
 
-    return PositionedTransition(
-      rect: animation.drive(positionTween),
+    return _FixedSizeSlidingTransition(
+      isLTR: isLTR,
+      offsetAnimation: animation.drive(anchorMovementInTransitionBox),
+      size: toBox.size,
       child: FadeTransition(
         opacity: fadeInFrom(0.25),
         child: DefaultTextStyle(

packages/flutter/lib/src/rendering/stack.dart

@@ -496,7 +496,7 @@ class RenderStack extends RenderBox
 
     child.layout(childConstraints, parentUsesSize: true);
 
-    late final double x;
+    final double x;
     if (childParentData.left != null) {
       x = childParentData.left!;
     } else if (childParentData.right != null) {
@@ -508,7 +508,7 @@ class RenderStack extends RenderBox
     if (x < 0.0 || x + child.size.width > size.width)
       hasVisualOverflow = true;
 
-    late final double y;
+    final double y;
     if (childParentData.top != null) {
       y = childParentData.top!;
     } else if (childParentData.bottom != null) {

packages/flutter/test/cupertino/nav_bar_transition_test.dart

@@ -171,6 +171,27 @@ void main() {
     );
   });
 
+  testWidgets('Bottom middle never changes size during the animation', (WidgetTester tester) async {
+    await tester.binding.setSurfaceSize(const Size(1080.0 / 2.75, 600));
+    addTearDown(() async {
+      await tester.binding.setSurfaceSize(const Size(800.0, 600.0));
+    });
+
+    await startTransitionBetween(
+      tester,
+      fromTitle: 'Page 1',
+    );
+
+    final Size size = tester.getSize(find.text('Page 1'));
+
+    for (int i = 0; i < 150; i++) {
+      await tester.pump(const Duration(milliseconds: 1));
+      expect(flying(tester, find.text('Page 1')), findsNWidgets(2));
+      expect(tester.getSize(flying(tester, find.text('Page 1')).first), size);
+      expect(tester.getSize(flying(tester, find.text('Page 1')).last), size);
+    }
+  });
+
   testWidgets('Bottom middle and top back label transitions their font', (WidgetTester tester) async {
     await startTransitionBetween(tester, fromTitle: 'Page 1');
 
@@ -978,6 +999,29 @@ void main() {
     );
   });
 
+  testWidgets('Top middle never changes size during the animation', (WidgetTester tester) async {
+    await tester.binding.setSurfaceSize(const Size(1080.0 / 2.75, 600));
+    addTearDown(() async {
+      await tester.binding.setSurfaceSize(const Size(800.0, 600.0));
+    });
+
+    await startTransitionBetween(
+      tester,
+      toTitle: 'Page 2',
+    );
+
+    Size? previousSize;
+
+    for (int i = 0; i < 150; i++) {
+      await tester.pump(const Duration(milliseconds: 1));
+      expect(flying(tester, find.text('Page 2')), findsOneWidget);
+      final Size size = tester.getSize(flying(tester, find.text('Page 2')));
+      if (previousSize != null)
+        expect(size, previousSize);
+      previousSize = size;
+    }
+  });
+
   testWidgets('Top middle fades in and slides in from the left in RTL', (WidgetTester tester) async {
     await startTransitionBetween(
       tester,