Make consecutive recorded frames while unpaused be a single undo action.

src/BizHawk.Client.Common/movie/tasproj/TasMovie.History.cs

@@ -28,6 +28,12 @@ namespace BizHawk.Client.Common
 		/// If not already recording a batch, does nothing.
 		/// </summary>
 		void EndBatch();
+
+		/// <summary>
+		/// Combine the last two undo actions, making them part of one batch action.
+		/// </summary>
+		void MergeLastActions();
+
 		/// <summary>
 		/// Undoes the most recent action batch, if any exist.
 		/// </summary>
@@ -178,6 +184,17 @@ namespace BizHawk.Client.Common
 			}
 		}
 
+		public void MergeLastActions()
+		{
+			Debug.Assert(UndoIndex + 1 == _history.Count, "Don't merge the last actions if they aren't the last actions.");
+			Debug.Assert(_history.Count > 1, "Don't merge when there aren't actions to merge.");
+
+			_history[_history.Count - 2].AddRange(_history[_history.Count - 1]);
+			_history.RemoveAt(_history.Count - 1);
+			Names.RemoveAt(Names.Count - 1);
+			UndoIndex--;
+		}
+
 		public void Undo()
 		{
 			if (UndoIndex == -1)

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -1041,6 +1041,7 @@ namespace BizHawk.Client.EmuHawk
 					InitializeFpsData();
 				}
 
+				if (value != _emulatorPaused) Tools.OnPauseToggle(value);
 				_emulatorPaused = value;
 			}
 		}

src/BizHawk.Client.EmuHawk/tools/TAStudio/TAStudio.IToolForm.cs

@@ -21,6 +21,8 @@ namespace BizHawk.Client.EmuHawk
 
 		private int _lastRefresh;
 		private bool _doPause;
+		private bool _extended;
+		private bool _extendNeedsMerge;
 
 		private void UpdateProgressBar()
 		{
@@ -55,7 +57,8 @@ namespace BizHawk.Client.EmuHawk
 
 		protected override void UpdateBefore()
 		{
-			if (CurrentTasMovie.IsAtEnd() && !CurrentTasMovie.IsRecording())
+			_extended = CurrentTasMovie.IsAtEnd() && !CurrentTasMovie.IsRecording();
+			if (_extended)
 			{
 				CurrentTasMovie.RecordFrame(CurrentTasMovie.Emulator.Frame, MovieSession.StickySource);
 			}
@@ -108,6 +111,18 @@ namespace BizHawk.Client.EmuHawk
 
 		protected override void FastUpdateAfter()
 		{
+			// Recording multiple frames, or auto-extending the movie, while unpaused should count as a single undo action.
+			// And do this before stopping the seek, which could puase.
+			if (!MainForm.EmulatorPaused && (CurrentTasMovie.IsRecording() || _extended))
+			{
+				if (_extendNeedsMerge) CurrentTasMovie.ChangeLog.MergeLastActions();
+				_extendNeedsMerge = true;
+			}
+			else
+			{
+				_extendNeedsMerge = false;
+			}
+
 			if (_seekingTo != -1 && Emulator.Frame >= _seekingTo)
 			{
 				StopSeeking();

src/BizHawk.Client.EmuHawk/tools/TAStudio/TAStudio.cs

@@ -890,6 +890,13 @@ namespace BizHawk.Client.EmuHawk
 			MainForm.TogglePause();
 		}
 
+		public override void OnPauseToggle(bool newPauseState)
+		{
+			// Consecutively recorded frames are merged into one undo action, until we pause.
+			// Then a new undo action should be used.
+			if (newPauseState) _extendNeedsMerge = false;
+		}
+
 		private void SetSplicer()
 		{
 			// TODO: columns selected?

src/BizHawk.Client.EmuHawk/tools/ToolFormBase.cs

@@ -136,5 +136,7 @@ namespace BizHawk.Client.EmuHawk
 		}
 
 		public virtual void HandleHotkeyUpdate() { }
+
+		public virtual void OnPauseToggle(bool newPauseState) { }
 	}
 }

src/BizHawk.Client.EmuHawk/tools/ToolManager.cs

@@ -804,6 +804,17 @@ namespace BizHawk.Client.EmuHawk
 			}
 		}
 
+		public void OnPauseToggle(bool newPauseState)
+		{
+			foreach (var tool in _tools)
+			{
+				if (tool.IsActive && tool is ToolFormBase toolForm)
+				{
+					toolForm.OnPauseToggle(newPauseState);
+				}
+			}
+		}
+
 		private static readonly IList<string> PossibleToolTypeNames = EmuHawk.ReflectionCache.Types.Select(t => t.AssemblyQualifiedName).ToList();
 
 		public bool IsAvailable(Type tool)