Fix crash deselecting moved pixels when using certain extensions (fix #5280)

Although the issue refers to deselecting MovingPixelsState, the same
crash could happen when canceling/finishing WritingTextState or
MovingSelectionState. This fixes the crash for all these states.

src/app/ui/editor/editor.cpp

@@ -58,6 +58,7 @@
 #include "app/util/tile_flags_utils.h"
 #include "base/chrono.h"
 #include "base/convert_to.h"
+#include "base/scoped_value.h"
 #include "doc/doc.h"
 #include "doc/mask_boundaries.h"
 #include "doc/slice.h"
@@ -266,6 +267,23 @@ void Editor::setStateInternal(const EditorStatePtr& newState)
 {
   m_brushPreview.hide();
 
+  // Some onLeaveState impls (like the ones from MovingPixelsState,
+  // WritingTextState, MovingSelectionState) might generate a
+  // Tx/Transaction::commit(), which will add a new undo state,
+  // triggering a sprite change scripting event
+  // (SpriteEvents::onAddUndoState). This event could be handled by an
+  // extension and that extension might want to save the current
+  // sprite (e.g. calling Sprite_saveCopyAs, the kind of extension
+  // that takes snapshots after each sprite change). That will be a
+  // new Context::executeCommand() for the save command, generating a
+  // BeforeCommandExecution signal, getting back to onLeaveState
+  // again. In that case, we just ignore the reentry as the first
+  // onLeaveState should handle everything (to avoid an stack
+  // overflow/infinite recursion).
+  if (m_leavingState)
+    return;
+  base::ScopedValue leaving(m_leavingState, true);
+
   // Fire before change state event, set the state, and fire after
   // change state event.
   EditorState::LeaveAction leaveAction = m_state->onLeaveState(this, newState.get());

src/app/ui/editor/editor.h

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018-2024  Igara Studio S.A.
+// Copyright (C) 2018-2025  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -458,6 +458,13 @@ private:
 
   DocView* m_docView;
 
+  // Special flag to avoid re-entering a new state when we are leaving
+  // the current one. This avoids an infinite onLeaveState() recursion
+  // in some special cases when an extension (third-party code)
+  // creates a new sprite change in the same sprite change scripting
+  // event.
+  bool m_leavingState = false;
+
   // Last known mouse position received by this editor when the
   // mouse button was pressed. Used for auto-scrolling. To get the
   // current mouse position on the editor you can use