Fixed rare soft-"freezes" with OnRender(c) { ...; InvalidateVisual(); } pattern (#18901)

src/Avalonia.Base/Media/MediaContext.Compositor.cs

@@ -1,4 +1,7 @@
+using System;
+using System.Diagnostics;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 
 using Avalonia.Platform;
@@ -11,16 +14,20 @@ namespace Avalonia.Media;
 
 partial class MediaContext
 {
-    private bool _scheduleCommitOnLastCompositionBatchCompletion;
-    
     /// <summary>
     /// Actually sends the current batch to the compositor and does the required housekeeping
     /// This is the only place that should be allowed to call Commit
     /// </summary>
     private CompositionBatch CommitCompositor(Compositor compositor)
     {
-        var commit = compositor.Commit();
+        // Compositor is allowed to schedule the next batch during Commit procedure if update
+        // was requested while it was doing an update
+        // This can e. g. happen if InvalidateVisual is called from Visual.OnRender which is currently separate
+        // from our FireInvokeOnRenderCallbacks loop, so we clean the commit request flag before rendering so
+        // it can be set again
         _requestedCommits.Remove(compositor);
+        var commit = compositor.Commit();
+       
         _pendingCompositionBatches[compositor] = commit;
         commit.Processed.ContinueWith(_ =>
             _dispatcher.Post(() => CompositionBatchFinished(compositor, commit), DispatcherPriority.Send),
@@ -43,9 +50,8 @@ partial class MediaContext
             _animationsAreWaitingForComposition = false;
 
             // Check if we have uncommited changes
-            if (_scheduleCommitOnLastCompositionBatchCompletion)
+            if (_requestedCommits.Count != 0)
             {
-                _scheduleCommitOnLastCompositionBatchCompletion = false;
                 if (!CommitCompositorsWithThrottling())
                     ScheduleRenderForAnimationsIfNeeded();
 
@@ -69,10 +75,10 @@ partial class MediaContext
     /// <returns>true if there are pending commits in-flight and there will be a "all-done" callback later</returns>
     private bool CommitCompositorsWithThrottling()
     {
+        Dispatcher.UIThread.VerifyAccess();
         // Check if we are still waiting for previous composition batches
         if (_pendingCompositionBatches.Count > 0)
         {
-            _scheduleCommitOnLastCompositionBatchCompletion = true;
             // Previous commit isn't handled yet
             return true;
         }
@@ -84,7 +90,6 @@ partial class MediaContext
         foreach (var c in _requestedCommits.ToArray())
             CommitCompositor(c);
         
-        _requestedCommits.Clear();
         return true;
     }