Fix graphical glitches while painting (fix #1262)

src/app/ui/editor/drawing_state.cpp

@@ -52,10 +52,6 @@ DrawingState::~DrawingState()
 
 void DrawingState::initToolLoop(Editor* editor, MouseMessage* msg)
 {
-  // It's needed to clear and redraw the brush boundaries after the
-  // first mouse pressed/point shape if drawn.
-  HideBrushPreview hide(editor->brushPreview());
-
   // Prepare preview image (the destination image will be our preview
   // in the tool-loop time, so we can see what we are drawing)
   editor->renderEngine().setPreviewImage(

src/app/ui/editor/standby_state.cpp

@@ -305,6 +305,12 @@ bool StandbyState::onMouseDown(Editor* editor, MouseMessage* msg)
     if (layerEdges)
       layerEdgesOption(false);
 
+    // We need to clear and redraw the brush boundaries after the
+    // first mouse pressed/point shape if drawn. This is to avoid
+    // graphical glitches (invalid areas in the ToolLoop's src/dst
+    // images).
+    HideBrushPreview hide(editor->brushPreview());
+
     tools::ToolLoop* toolLoop = create_tool_loop(editor, context);
     if (toolLoop) {
       EditorStatePtr newState(new DrawingState(toolLoop));

src/app/ui/editor/tool_loop_impl.cpp

@@ -242,10 +242,8 @@ public:
   }
 
   void updateDirtyArea() override {
-    // TODO find a way to avoid calling hide/show brush preview here
-    HideBrushPreview hide(m_editor->brushPreview());
-    m_document->notifySpritePixelsModified(m_sprite, m_dirtyArea,
-                                           m_frame);
+    m_document->notifySpritePixelsModified(
+      m_sprite, m_dirtyArea, m_frame);
   }
 
   void updateStatusBar(const char* text) override {