LibGUI: Only redraw the dirty rect in GWidget.

There is some trouble here with the asynchronous nature of WindowServer
and the single per-window backing store we're drawing into. If we start
repainting a widget with a pending invalidation, that invalidation might
get flushed by the WindowServer mid-paint.

3 files changed, 9 insertions, 7 deletions

diff --git a/LibGUI/GWidget.cpp b/LibGUI/GWidget.cpp
index 62caf4cc7f..57236f363f 100644
--- a/LibGUI/GWidget.cpp
+++ b/LibGUI/GWidget.cpp
@@ -61,7 +61,7 @@ void GWidget::paintEvent(GPaintEvent& event)
 {
     if (fillWithBackgroundColor()) {
         Painter painter(*this);
-        painter.fill_rect(rect(), backgroundColor());
+        painter.fill_rect(event.rect(), backgroundColor());
     }
     for (auto* ch : children()) {
         auto* child = (GWidget*)ch;
diff --git a/LibGUI/GWindow.cpp b/LibGUI/GWindow.cpp
index 93d2dd9c54..53899e45ec 100644
--- a/LibGUI/GWindow.cpp
+++ b/LibGUI/GWindow.cpp
@@ -102,12 +102,12 @@ void GWindow::event(GEvent& event)
         if (!m_main_widget)
             return;
         auto& paint_event = static_cast<GPaintEvent&>(event);
-        if (paint_event.rect().is_empty()) {
-            m_main_widget->paintEvent(*make<GPaintEvent>(m_main_widget->rect()));
-        } else {
-            m_main_widget->event(event);
-        }
-        int rc = gui_invalidate_window(m_window_id, nullptr);
+        auto rect = paint_event.rect();
+        if (rect.is_empty())
+            rect = m_main_widget->rect();
+        m_main_widget->event(*make<GPaintEvent>(rect));
+        GUI_Rect gui_rect = rect;
+        int rc = gui_invalidate_window(m_window_id, &gui_rect);
         ASSERT(rc == 0);
     }
 
diff --git a/LibGUI/GWindow.h b/LibGUI/GWindow.h
index ece4b14cc3..08e79daea6 100644
--- a/LibGUI/GWindow.h
+++ b/LibGUI/GWindow.h
@@ -37,6 +37,8 @@ public:
 
     void close();
 
+    GWidget* main_widget() { return m_main_widget; }
+    const GWidget* main_widget() const { return m_main_widget; }
     void set_main_widget(GWidget*);
 
     GraphicsBitmap* backing() { return m_backing.ptr(); }