fix(chat): keep long-press from missing in fast-moving chat (#579)

Freeze the message list on a stationary touch-and-hold (pointer-counted,
touch-slop aware) so an incoming message can't shift the row out from
under a long-press, while still allowing scroll to cancel the freeze.

Co-authored-by: Francis Lavoie <lavofr@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/screens/channel/chat/chat_interaction_pause_test.dart

@@ -0,0 +1,86 @@
+import 'package:flutter/gestures.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frosty/screens/channel/chat/stores/chat_interaction_pause.dart';
+
+void main() {
+  late ChatInteractionPause pause;
+
+  setUp(() => pause = ChatInteractionPause());
+
+  // A move clearly past the touch slop (a scroll), and one well within it.
+  final beyondSlop = Offset(0, kTouchSlop + 10);
+  final withinSlop = Offset(0, kTouchSlop - 5);
+
+  test('is not paused initially', () {
+    expect(pause.isPaused, isFalse);
+  });
+
+  test('is paused while a stationary pointer is down (long-press candidate)', () {
+    pause.pointerDown(1, Offset.zero);
+
+    expect(pause.isPaused, isTrue);
+  });
+
+  test('resumes once the pointer is lifted', () {
+    pause.pointerDown(1, Offset.zero);
+    pause.pointerUp(1);
+
+    expect(pause.isPaused, isFalse);
+  });
+
+  test('resumes when the pointer moves beyond the touch slop (scrolling)', () {
+    pause.pointerDown(1, Offset.zero);
+    pause.pointerMove(1, beyondSlop);
+
+    // The finger is dragging — this is a scroll, not a hold, so the chat must
+    // keep flowing rather than freeze.
+    expect(pause.isPaused, isFalse);
+  });
+
+  test('stays paused for tiny jitter within the touch slop', () {
+    pause.pointerDown(1, Offset.zero);
+    pause.pointerMove(1, withinSlop);
+
+    expect(pause.isPaused, isTrue);
+  });
+
+  test('stays resumed for the rest of the gesture even if the finger drifts '
+      'back within slop', () {
+    pause.pointerDown(1, Offset.zero);
+    pause.pointerMove(1, beyondSlop);
+    pause.pointerMove(1, withinSlop);
+
+    // Once a gesture is recognized as a scroll it should not flip back to a
+    // hold mid-drag.
+    expect(pause.isPaused, isFalse);
+  });
+
+  test('stays paused until every stationary pointer lifts (multi-touch)', () {
+    pause.pointerDown(1, Offset.zero);
+    pause.pointerDown(2, Offset.zero);
+    pause.pointerUp(1);
+
+    expect(pause.isPaused, isTrue);
+
+    pause.pointerUp(2);
+    expect(pause.isPaused, isFalse);
+  });
+
+  test('a stray pointer-up never leaves the gate stuck', () {
+    pause.pointerUp(99);
+
+    expect(pause.isPaused, isFalse);
+
+    pause.pointerDown(1, Offset.zero);
+    expect(pause.isPaused, isTrue);
+  });
+
+  test('reset clears all active pointers', () {
+    pause.pointerDown(1, Offset.zero);
+    pause.pointerDown(2, Offset.zero);
+
+    pause.reset();
+
+    expect(pause.isPaused, isFalse);
+  });
+}