InputDecorator - animate between transparent hoverColor and hoverColor (#168244)

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As described in https://github.com/flutter/flutter/issues/155169,
TextFields with hoverColor set can have an issue where they flicker as
the mouse enters and exits their bounds.

_BorderContainerState uses a 15ms animation to transition between
non-hovered and hovered states. Part of this animation is a ColorTween
between Colors.transparent and a given hoverColor. Critically,
Colors.transparent is transparent black. On high refresh rate displays
(any display with faster than 60z), this can result in a frame being
rendered where the TextField fill color is a dark, semi-transparent
version of hoverColor.

The immediate solution to this is to animate between hoverColor and a
hoverColor with alpha 0.

Before:
https://github.com/user-attachments/assets/01d62c6a-015d-4dc7-9897-7c3a75ca8aa0
After:
https://github.com/user-attachments/assets/ff51a5b2-ba4a-4508-a111-4c915d31ac46

Fixes https://github.com/flutter/flutter/issues/155169
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---------

Co-authored-by: Nick Sparks <6109599+nsparks@users.noreply.github.com>

packages/flutter/lib/src/material/input_decorator.dart

@@ -218,7 +218,9 @@ class _BorderContainerState extends State<_BorderContainer> with TickerProviderS
     );
     _border = _InputBorderTween(begin: widget.border, end: widget.border);
     _hoverAnimation = CurvedAnimation(parent: _hoverColorController, curve: Curves.linear);
-    _hoverColorTween = ColorTween(begin: Colors.transparent, end: widget.hoverColor);
+
+    // Animate between transparent [widget.hoverColor] and [widget.hoverColor].
+    _hoverColorTween = ColorTween(begin: widget.hoverColor.withAlpha(0), end: widget.hoverColor);
   }
 
   @override
@@ -240,7 +242,8 @@ class _BorderContainerState extends State<_BorderContainer> with TickerProviderS
         ..forward();
     }
     if (widget.hoverColor != oldWidget.hoverColor) {
-      _hoverColorTween = ColorTween(begin: Colors.transparent, end: widget.hoverColor);
+      // Animate between transparent [widget.hoverColor] and [widget.hoverColor].
+      _hoverColorTween = ColorTween(begin: widget.hoverColor.withAlpha(0), end: widget.hoverColor);
     }
     if (widget.isHovering != oldWidget.isHovering) {
       if (widget.isHovering) {

packages/flutter/test/material/input_decorator_test.dart

@@ -238,6 +238,14 @@ Rect getContainerRect(WidgetTester tester) {
   return box.paintBounds;
 }
 
+Animation<double> _getHoverAnimation(WidgetTester tester) {
+  final CustomPaint customPaint = tester.widget(findBorderPainter());
+  final dynamic /*_InputBorderPainter*/ inputBorderPainter = customPaint.foregroundPainter;
+  // ignore: avoid_dynamic_calls
+  final Animation<double> animation = inputBorderPainter.hoverAnimation as Animation<double>;
+  return animation;
+}
+
 InputBorder? getBorder(WidgetTester tester) {
   if (!tester.any(findBorderPainter())) {
     return null;
@@ -13361,6 +13369,15 @@ void main() {
       await tester.pump(const Duration(milliseconds: 15));
       expect(getContainerColor(tester), isSameColorAs(fillColor));
 
+      // Test that for high refresh rate displays, the color mid-animation is somewhere between
+      // the fill color and the hover color.
+      await pumpDecorator(hovering: true);
+      expect(getContainerColor(tester), isSameColorAs(fillColor));
+      await tester.pump(const Duration(milliseconds: 6));
+      final Color midHoverColor =
+          Color.lerp(hoverColor.withAlpha(0), hoverColor, _getHoverAnimation(tester).value)!;
+      expect(getContainerColor(tester), isSameColorAs(Color.alphaBlend(midHoverColor, fillColor)));
+
       await pumpDecorator(hovering: false, enabled: false);
       expect(getContainerColor(tester), isSameColorAs(disabledColor));
       await tester.pump(const Duration(seconds: 10));