button: Replace GestureDetector with Listener for AnimatedScaleOnTap

Fixes #1953.
Latter provides instant scaleDown animation on tap,
unlike GestureDetector which has a `kPressTimeOut` causing
delay of 100ms
    https://main-api.flutter.dev/flutter/gestures/kPressTimeout-constant.html

Author: MritunjayTiwari14

lib/widgets/button.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 
 import 'color.dart';
@@ -159,7 +160,7 @@ class ZulipWebUiKitButton extends StatelessWidget {
 
     final labelColor = _labelColor(designVariables);
 
-    return AnimatedScaleOnTap(
+    return AnimatedScaleOnPrimaryPointerDown(
       scaleEnd: 0.96,
       duration: Duration(milliseconds: 100),
       child: TextButton.icon(
@@ -270,10 +271,10 @@ class ZulipIconButton extends StatelessWidget {
   }
 }
 
-/// Apply [Transform.scale] to the child widget when tapped, and reset its scale
-/// when released, while animating the transitions.
-class AnimatedScaleOnTap extends StatefulWidget {
-  const AnimatedScaleOnTap({
+/// Apply [Transform.scale] to the child widget on primary pointer-down,
+/// and reset its scale on -up or -cancel, with animated transitions.
+class AnimatedScaleOnPrimaryPointerDown extends StatefulWidget {
+  const AnimatedScaleOnPrimaryPointerDown({
     super.key,
     required this.scaleEnd,
     required this.duration,
@@ -289,11 +290,12 @@ class AnimatedScaleOnTap extends StatefulWidget {
   final Widget child;
 
   @override
-  State<AnimatedScaleOnTap> createState() => _AnimatedScaleOnTapState();
+  State<AnimatedScaleOnPrimaryPointerDown> createState() => _AnimatedScaleOnPrimaryPointerDownState();
 }
 
-class _AnimatedScaleOnTapState extends State<AnimatedScaleOnTap> {
+class _AnimatedScaleOnPrimaryPointerDownState extends State<AnimatedScaleOnPrimaryPointerDown> {
   double _scale = 1;
+  int _pressedButton = 0;
 
   void _changeScale(double scale) {
     setState(() {
@@ -303,11 +305,19 @@ class _AnimatedScaleOnTapState extends State<AnimatedScaleOnTap> {
 
   @override
   Widget build(BuildContext context) {
-    return GestureDetector(
+    return Listener(
       behavior: HitTestBehavior.translucent,
-      onTapDown: (_) =>  _changeScale(widget.scaleEnd),
-      onTapUp: (_) =>    _changeScale(1),
-      onTapCancel: () => _changeScale(1),
+      onPointerDown: (PointerDownEvent pointerDownEvent) {
+        _pressedButton = pointerDownEvent.buttons;
+        if(_pressedButton & kPrimaryButton != 0) _changeScale(widget.scaleEnd);
+      },
+      onPointerUp: (_) {
+        if(_pressedButton & kPrimaryButton != 0) _changeScale(1);
+      },
+      onPointerCancel: (_) {
+        if(_pressedButton & kPrimaryButton != 0) _changeScale(1);
+        _pressedButton = 0;
+      },
       child: AnimatedScale(
         scale: _scale,
         duration: widget.duration,

lib/widgets/home.dart

@@ -287,7 +287,7 @@ class _NavigationBarButton extends StatelessWidget {
     final designVariables = DesignVariables.of(context);
     final color = selected ? designVariables.iconSelected : designVariables.icon;
 
-    Widget result = AnimatedScaleOnTap(
+    Widget result = AnimatedScaleOnPrimaryPointerDown(
       scaleEnd: 0.875,
       duration: const Duration(milliseconds: 100),
       child: Material(
@@ -389,7 +389,7 @@ void _showMainMenu(BuildContext context, {
                   child: Column(children: menuItems)))),
               const Padding(
                 padding: EdgeInsets.symmetric(horizontal: 16),
-                child: AnimatedScaleOnTap(
+                child: AnimatedScaleOnPrimaryPointerDown(
                   scaleEnd: 0.95,
                   duration: Duration(milliseconds: 100),
                   child: BottomSheetDismissButton(
@@ -459,7 +459,7 @@ abstract class _MenuButton extends StatelessWidget {
         ~WidgetState.pressed: selected ? borderSideSelected : null,
       }));
 
-    return AnimatedScaleOnTap(
+    return AnimatedScaleOnPrimaryPointerDown(
       duration: const Duration(milliseconds: 100),
       scaleEnd: 0.95,
       child: ConstrainedBox(

test/widgets/button_test.dart

@@ -114,6 +114,39 @@ void main() {
       check(renderObject).size.equals(Size.square(40));
     });
 
+    group('AnimatedScaleOnTap', () {
+      void checkScale(WidgetTester tester, Finder finder, double expectedScale) {
+        final scale = tester.widget<AnimatedScale>(finder).scale;
+        check(scale).equals(expectedScale);
+      }
+
+      testWidgets('Animation happen instantly when tap down', (tester) async {
+        addTearDown(testBinding.reset);
+
+        await tester.pumpWidget(TestZulipApp(
+          child: AnimatedScaleOnPrimaryPointerDown(
+            scaleEnd: 0.95,
+            duration: Duration(milliseconds: 100),
+            child: UnconstrainedBox(
+              child: ZulipIconButton(
+                icon: ZulipIcons.follow,
+                onPressed: () {})))));
+        await tester.pump();
+
+        final animatedScaleFinder = find.byType(AnimatedScale);
+
+        // Now that the widget is being held down, its scale should be at the target scaleEnd i.e 0.95.
+        final gesture = await tester.startGesture(tester.getCenter(find.byType(ZulipIconButton)));
+        await tester.pumpAndSettle(const Duration(milliseconds: 50));
+        checkScale(tester, animatedScaleFinder, 0.95);
+
+        // After releasing, the scale must return to 1.0.
+        await gesture.up();
+        await tester.pumpAndSettle();
+        checkScale(tester, animatedScaleFinder, 1.0);
+      });
+    });
+
     // TODO test that the touch feedback fills the whole square
   });
 }