After moving a window, try to repaint a bit less.

Only repaint windows that intersect either the old or the new rect.
Also only repaint those rects in the root widget.

Widgets/Event.h

@@ -2,6 +2,7 @@
 
 #include <AK/Types.h>
 #include "Point.h"
+#include "Rect.h"
 
 static const char* eventNames[] = {
     "Invalid",
@@ -61,10 +62,16 @@ public:
 
 class PaintEvent final : public Event {
 public:
-    PaintEvent()
+    explicit PaintEvent(const Rect& rect = Rect())
         : Event(Event::Paint)
+        , m_rect(rect)
     {
     }
+
+    const Rect& rect() const { return m_rect; }
+private:
+    friend class WindowManager;
+    Rect m_rect;
 };
 
 class ShowEvent final : public Event {

Widgets/Rect.h

@@ -67,6 +67,14 @@ public:
         setY(top);
     }
 
+    bool intersects(const Rect& other) const
+    {
+        return left() < other.right()
+            && other.left() < right()
+            && top() < other.bottom()
+            && other.top() < bottom();
+    }
+
     int x() const { return location().x(); }
     int y() const { return location().y(); }
     int width() const { return m_width; }

Widgets/RootWidget.cpp

@@ -13,10 +13,15 @@ RootWidget::~RootWidget()
 
 void RootWidget::onPaint(PaintEvent& event)
 {
-    //printf("RootWidget::onPaint\n");
-    Painter painter(*this);
-    painter.fillRect(Rect(0, 0, 800, 600), Color(0x40, 0x40, 0x40));
     Widget::onPaint(event);
+
+    printf("RootWidget::onPaint: %d,%d %dx%d\n",
+            event.rect().x(),
+            event.rect().y(),
+            event.rect().width(),
+            event.rect().height());
+    Painter painter(*this);
+    painter.fillRect(event.rect(), Color(0x40, 0x40, 0x40));
 }
 
 void RootWidget::onMouseMove(MouseEvent& event)

Widgets/Widget.cpp

@@ -23,6 +23,11 @@ void Widget::setWindowRelativeRect(const Rect& rect)
     update();
 }
 
+void Widget::repaint(const Rect& rect)
+{
+    event(*make<PaintEvent>(rect));
+}
+
 void Widget::event(Event& event)
 {
     switch (event.type()) {
@@ -94,7 +99,7 @@ void Widget::update()
     if (m_hasPendingPaintEvent)
         return;
     m_hasPendingPaintEvent = true;
-    EventLoop::main().postEvent(this, make<PaintEvent>());
+    EventLoop::main().postEvent(this, make<PaintEvent>(rect()));
 }
 
 Widget::HitTestResult Widget::hitTest(int x, int y)

Widgets/Widget.h

@@ -34,6 +34,7 @@ public:
     Rect rect() const { return { 0, 0, width(), height() }; }
 
     void update();
+    void repaint(const Rect&);
 
     struct HitTestResult {
         Widget* widget { nullptr };

Widgets/Window.cpp

@@ -41,6 +41,11 @@ void Window::setRect(const Rect& rect)
     WindowManager::the().notifyRectChanged(*this, oldRect, m_rect);
 }
 
+void Window::repaint()
+{
+    event(*make<PaintEvent>());
+}
+
 void Window::event(Event& event)
 {
     if (event.isMouseEvent()) {
@@ -58,12 +63,23 @@ void Window::event(Event& event)
     }
 
     if (event.isPaintEvent()) {
+        auto& pe = static_cast<PaintEvent&>(event);
+        printf("Window[\"%s\"]: paintEvent %d,%d %dx%d\n", className(),
+                pe.rect().x(),
+                pe.rect().y(),
+                pe.rect().width(),
+                pe.rect().height());
+
         if (isBeingDragged()) {
             // Ignore paint events during window drag.
             return;
         }
-        if (m_mainWidget)
-            return m_mainWidget->event(event);
+        if (m_mainWidget) {
+            if (pe.rect().isEmpty())
+                return m_mainWidget->event(*make<PaintEvent>(m_mainWidget->rect()));
+            else
+                return m_mainWidget->event(event);
+        }
         return Object::event(event);
     }
 

Widgets/Window.h

@@ -35,6 +35,8 @@ public:
     bool isBeingDragged() const { return m_isBeingDragged; }
     void setIsBeingDragged(bool b) { m_isBeingDragged = b; }
 
+    void repaint();
+
 private:
     String m_title;
     Rect m_rect;

Widgets/WindowManager.cpp

@@ -21,6 +21,23 @@ static inline Rect titleBarRectForWindow(const Window& window)
     };
 }
 
+static inline Rect borderRectForWindow(const Window& window)
+{
+    auto titleBarRect = titleBarRectForWindow(window);
+    return { titleBarRect.x() - 1, 
+        titleBarRect.y() - 1,
+        titleBarRect.width() + 2,
+        windowFrameWidth + windowTitleBarHeight + window.rect().height() + 4
+    };
+}
+
+static inline Rect outerRectForWindow(const Window& window)
+{
+    auto rect = borderRectForWindow(window);
+    rect.inflate(2, 2);
+    return rect;
+}
+
 WindowManager& WindowManager::the()
 {
     static WindowManager* s_the = new WindowManager;
@@ -49,7 +66,9 @@ void WindowManager::paintWindowFrame(Window& window)
 
     //printf("[WM] paintWindowFrame {%p}, rect: %d,%d %dx%d\n", &window, window.rect().x(), window.rect().y(), window.rect().width(), window.rect().height());
 
-    Rect topRect = titleBarRectForWindow(window);
+    auto titleBarRect = titleBarRectForWindow(window);
+    auto outerRect = outerRectForWindow(window);
+    auto borderRect = borderRectForWindow(window);
 
     Rect bottomRect {
         window.x() - windowFrameWidth,
@@ -71,12 +90,6 @@ void WindowManager::paintWindowFrame(Window& window)
         window.height()
     };
 
-    Rect borderRect {
-        topRect.x() - 1, 
-        topRect.y() - 1,
-        topRect.width() + 2,
-        windowFrameWidth + windowTitleBarHeight + window.rect().height() + 4
-    };
 
     if (!m_lastDragRect.isEmpty()) {
         p.xorRect(m_lastDragRect, Color(255, 0, 0));
@@ -84,22 +97,20 @@ void WindowManager::paintWindowFrame(Window& window)
     }
 
     if (m_dragWindow == &window) {
-        borderRect.inflate(2, 2);
-        p.xorRect(borderRect, Color(255, 0, 0));
-        m_lastDragRect = borderRect;
+        p.xorRect(outerRect, Color(255, 0, 0));
+        m_lastDragRect = outerRect;
         return;
     }
 
     p.drawRect(borderRect, Color(255, 255, 255));
-    borderRect.inflate(2, 2);
-    p.drawRect(borderRect, m_windowBorderColor);
+    p.drawRect(outerRect, m_windowBorderColor);
 
-    p.fillRect(topRect, m_windowBorderColor);
+    p.fillRect(titleBarRect, m_windowBorderColor);
     p.fillRect(bottomRect, m_windowBorderColor);
     p.fillRect(leftRect, m_windowBorderColor);
     p.fillRect(rightRect, m_windowBorderColor);
 
-    p.drawText(topRect, window.title(), Painter::TextAlignment::Center, m_windowTitleColor);
+    p.drawText(titleBarRect, window.title(), Painter::TextAlignment::Center, m_windowTitleColor);
 }
 
 void WindowManager::addWindow(Window& window)
@@ -124,6 +135,7 @@ void WindowManager::handleTitleBarMouseEvent(Window& window, MouseEvent& event)
         m_dragWindow = &window;
         m_dragOrigin = event.position();
         m_dragWindowOrigin = window.position();
+        m_dragStartRect = outerRectForWindow(window);
         window.setIsBeingDragged(true);
         return;
     }
@@ -136,14 +148,38 @@ void WindowManager::handleTitleBarMouseEvent(Window& window, MouseEvent& event)
 #endif
 }
 
+void WindowManager::repaintAfterMove(const Rect& oldRect, const Rect& newRect)
+{
+    printf("[WM] repaint: [%d,%d %dx%d] -> [%d,%d %dx%d]\n",
+            oldRect.x(),
+            oldRect.y(),
+            oldRect.width(),
+            oldRect.height(),
+            newRect.x(),
+            newRect.y(),
+            newRect.width(),
+            newRect.height());
+
+    m_rootWidget->repaint(oldRect);
+    m_rootWidget->repaint(newRect);
+    for (auto* window : m_windows) {
+        if (outerRectForWindow(*window).intersects(oldRect) || outerRectForWindow(*window).intersects(newRect)) {
+            paintWindowFrame(*window);
+            window->repaint();
+        }
+    }
+}
+
 void WindowManager::processMouseEvent(MouseEvent& event)
 {
     if (event.type() == Event::MouseUp) {
         if (m_dragWindow) {
             printf("[WM] Finish dragging Window{%p}\n", m_dragWindow);
             m_dragWindow->setIsBeingDragged(false);
+            m_dragEndRect = outerRectForWindow(*m_dragWindow);
             m_dragWindow = nullptr;
-            EventLoop::main().postEvent(this, make<PaintEvent>());
+
+            repaintAfterMove(m_dragStartRect, m_dragEndRect);
             return;
         }
     }
@@ -179,6 +215,11 @@ void WindowManager::processMouseEvent(MouseEvent& event)
 void WindowManager::handlePaintEvent(PaintEvent& event)
 {
     //printf("[WM] paint event\n");
+    if (event.rect().isEmpty()) {
+        event.m_rect.setWidth(AbstractScreen::the().width());
+        event.m_rect.setHeight(AbstractScreen::the().height());
+    }
+
     m_rootWidget->event(event);
     paintWindowFrames();
 

Widgets/WindowManager.h

@@ -29,7 +29,8 @@ private:
     void processMouseEvent(MouseEvent&);
     void handleTitleBarMouseEvent(Window&, MouseEvent&);
     void handlePaintEvent(PaintEvent&);
-
+    void repaintAfterMove(const Rect& oldRect, const Rect& newRect);
+    
     virtual void event(Event&) override;
 
     Color m_windowBorderColor;
@@ -43,4 +44,6 @@ private:
     Point m_dragOrigin;
     Point m_dragWindowOrigin;
     Rect m_lastDragRect;
+    Rect m_dragStartRect;
+    Rect m_dragEndRect;
 };