Have WindowManager::invalidate() unite dirty rects that intersect each other.

This is kinda primitive but it avoids double-draw when slowly dragging stuff.

Widgets/Rect.cpp

@@ -20,3 +20,13 @@ void Rect::intersect(const Rect& other)
     m_size.setWidth((r - l) + 1);
     m_size.setHeight((b - t) + 1);
 }
+
+Rect Rect::united(const Rect& other) const
+{
+    Rect rect;
+    rect.set_left(min(left(), other.left()));
+    rect.set_top(min(top(), other.top()));
+    rect.set_right(max(right(), other.right()));
+    rect.set_bottom(max(bottom(), other.bottom()));
+    return rect;
+}

Widgets/Rect.h

@@ -88,12 +88,12 @@ public:
 
     void set_right(int right)
     {
-        setWidth(right - x());
+        setWidth(right - x() + 1);
     }
 
     void set_bottom(int bottom)
     {
-        setHeight(bottom - y());
+        setHeight(bottom - y() + 1);
     }
 
     bool intersects(const Rect& other) const
@@ -132,6 +132,8 @@ public:
         return r;
     }
 
+    Rect united(const Rect&) const;
+
 private:
     Point m_location;
     Size m_size;

Widgets/WindowManager.cpp

@@ -8,6 +8,8 @@
 #include "Process.h"
 #include "MemoryManager.h"
 
+//#define DEBUG_FLUSH_YELLOW
+
 static const int windowTitleBarHeight = 16;
 
 static inline Rect titleBarRectForWindow(const Rect& window)
@@ -327,6 +329,12 @@ void WindowManager::invalidate(const Rect& a_rect)
     for (auto& r : m_invalidated_rects) {
         if (r.contains(rect))
             return;
+        if (r.intersects(rect)) {
+            // Unite with the existing dirty rect.
+            // FIXME: It would be much nicer to compute the exact rects needing repaint.
+            r = r.united(rect);
+            return;
+        }
     }
 
     m_invalidated_rects.append(rect);
@@ -350,6 +358,11 @@ void WindowManager::flush(const Rect& a_rect)
     const RGBA32* back_ptr = m_back_bitmap->scanline(rect.y()) + rect.x();
     size_t pitch = m_back_bitmap->pitch();
 
+#ifdef DEBUG_FLUSH_YELLOW
+    Painter p(*m_front_bitmap);
+    p.fill_rect(rect, Color::Yellow);
+#endif
+
     for (int y = 0; y < rect.height(); ++y) {
         fast_dword_copy(front_ptr, back_ptr, rect.width());
         front_ptr = (RGBA32*)((byte*)front_ptr + pitch);