fix(player): tick the frame-clock keep-alive only on Linux

The 1x1 keep-alive repaint loop was extended to Windows in a87aa296 to
paper over the legacy compositing path's resize desync (#227); the DComp
rework replaced that presentation path entirely. On the DComp engine the
100ms repaints become DirectComposition commits during playback, and
once fullscreen focus engages VRR (FreeSync/G-Sync) every commit forces
a scanout off the video's cadence - the micro-stutter of #1707.

The widget now owns the platform decision behind a test seam, and a new
quiescence test pins the hidden-chrome player UI to zero scheduled
frames so no future ticker can silently reintroduce the defect.

lib/widgets/video_controls/video_controls.dart

@@ -1021,10 +1021,11 @@ class _PlexVideoControlsState extends State<PlexVideoControls>
               onHover: (_) => _showControlsFromPointerActivity(),
               child: Stack(
                 children: [
-                  // Keep-alive: 1px widget that continuously repaints to prevent
-                  // Flutter animations from freezing when the frame clock goes idle
-                  if (Platform.isLinux || Platform.isWindows)
-                    const Positioned(top: 0, left: 0, child: LinuxKeepAlive()),
+                  // Keep-alive for Linux's idle GTK frame clock; inert on every
+                  // other platform (the widget owns the platform decision).
+                  // Windows must NOT tick here: forced repaints during playback
+                  // perturb VRR scanout in fullscreen (#1707).
+                  const Positioned(top: 0, left: 0, child: LinuxKeepAlive()),
                   // Also handles long-press for 2x speed.
                   Positioned.fill(
                     child: Semantics(

lib/widgets/video_controls/widgets/linux_keep_alive.dart

@@ -1,12 +1,30 @@
 import 'dart:async' show Timer;
+import 'dart:io' show Platform;
 
 import 'package:flutter/material.dart';
 
-/// A 1x1 pixel widget that continuously repaints to keep Flutter's frame clock active on Linux.
-/// This prevents animations from freezing when GTK's frame clock goes idle.
+/// A 1x1 pixel widget that continuously repaints to keep Flutter's frame clock
+/// active on Linux, where GTK's frame clock goes idle and freezes animations.
+///
+/// Linux-only by design. Mounting this on other platforms is harmful: on
+/// Windows the 10Hz repaints become DirectComposition commits during playback,
+/// and once fullscreen focus engages VRR (FreeSync/G-Sync) each commit forces a
+/// scanout off the video's cadence — the micro-stutter of issue #1707. Every
+/// non-Linux platform must instead uphold the invariant that the player UI
+/// schedules no frames while its chrome is hidden.
 class LinuxKeepAlive extends StatefulWidget {
   const LinuxKeepAlive({super.key});
 
+  /// Forces the platform decision so tests can exercise both the ticking and
+  /// the inert path regardless of host OS.
+  @visibleForTesting
+  static bool? debugIsLinuxOverride;
+
+  /// Whether this widget repaints on the current platform. Exposed so the
+  /// quiescence test can pin the policy to exactly [Platform.isLinux].
+  @visibleForTesting
+  static bool get ticksOnThisPlatform => debugIsLinuxOverride ?? Platform.isLinux;
+
   @override
   State<LinuxKeepAlive> createState() => _LinuxKeepAliveState();
 }
@@ -18,6 +36,7 @@ class _LinuxKeepAliveState extends State<LinuxKeepAlive> {
   @override
   void initState() {
     super.initState();
+    if (!LinuxKeepAlive.ticksOnThisPlatform) return;
     // Repaint every 100ms to keep Flutter's frame scheduler active.
     _timer = Timer.periodic(const Duration(milliseconds: 100), (_) {
       if (mounted) {
@@ -36,6 +55,7 @@ class _LinuxKeepAliveState extends State<LinuxKeepAlive> {
 
   @override
   Widget build(BuildContext context) {
+    if (_timer == null) return const SizedBox.shrink();
     return SizedBox(width: 1, height: 1, child: ColoredBox(color: Color.fromRGBO(0, 0, 0, _tick % 2 == 0 ? 0.1 : 0.2)));
   }
 }