lightbox: Apply correct fix for slider flickering

This changes makes it so that the async function `seekTo` is awaited
before the `setState` in `Slider.onChangeEnd`, thus first invoking
the `ValueNotifier.value` setter which as a side-effect calls the
`setState` in `_handleVideoControllerUpdate`, and then the `setState`
in `Slider.onChangeEnd`.

Resulting in both setState calls to be coalesced in a single redraw.
Whereas previously the unawaited `seekTo` call would delay the
`ValueNotifier.value` setter call in a microtask.

Author: rajveermalviya

lib/widgets/lightbox.dart

@@ -277,20 +277,7 @@ class _VideoPositionSliderControlState extends State<_VideoPositionSliderControl
   }
 
   void _handleVideoControllerUpdate() {
-    setState(() {
-      // After 'controller.seekTo' is called in 'Slider.onChangeEnd' the
-      // position indicator switches back to the actual controller's position
-      // but since the call 'seekTo' completes before the actual controller
-      // updates are notified, the position indicator that switches to controller's
-      // position can show the older position before the call to 'seekTo' for a
-      // single frame, resulting in a glichty UX.
-      //
-      // To avoid that, we delay the position indicator switch from '_sliderValue' to
-      // happen when we are notified of the controller update.
-      if (_isSliderDragging && _sliderValue == widget.controller.value.position) {
-        _isSliderDragging = false;
-      }
-    });
+    setState(() {});
   }
 
   static String _formatDuration(Duration value) {
@@ -325,15 +312,15 @@ class _VideoPositionSliderControlState extends State<_VideoPositionSliderControl
               _sliderValue = Duration(milliseconds: value.toInt());
             });
           },
-          onChangeEnd: (value) {
+          onChangeEnd: (value) async {
             final durationValue = Duration(milliseconds: value.toInt());
-            setState(() {
-              _sliderValue = durationValue;
-            });
-            widget.controller.seekTo(durationValue);
-
-            // The toggling back of '_isSliderDragging' is omitted here intentionally,
-            // see '_handleVideoControllerUpdates'.
+            await widget.controller.seekTo(durationValue);
+            if (mounted) {
+              setState(() {
+                _sliderValue = durationValue;
+                _isSliderDragging = false;
+              });
+            }
           },
         ),
       ),