LibGUI: Add Widget::repaint() to force an immediate repaint

In most situations, Widget::update() is preferable, since that allows us
to coalesce repaints and avoid redundant work, reducing system load.

However, there are some cases where you really want a paint to happen
right away, to make sure that the user has a chance to see a short-lived
visual state.

Userland/Libraries/LibGUI/Widget.cpp

@@ -616,6 +616,22 @@ void Widget::update(const Gfx::IntRect& rect)
         window->update(bound_by_widget.translated(window_relative_rect().location()));
 }
 
+void Widget::repaint()
+{
+    if (rect().is_empty())
+        return;
+    repaint(rect());
+}
+
+void Widget::repaint(Gfx::IntRect const& rect)
+{
+    auto* window = this->window();
+    if (!window)
+        return;
+    update(rect);
+    window->flush_pending_paints_immediately();
+}
+
 Gfx::IntRect Widget::window_relative_rect() const
 {
     auto rect = relative_rect();

Userland/Libraries/LibGUI/Widget.h

@@ -136,9 +136,14 @@ public:
     Gfx::IntRect rect() const { return { 0, 0, width(), height() }; }
     Gfx::IntSize size() const { return m_relative_rect.size(); }
 
+    // Invalidate the widget (or an area thereof), causing a repaint to happen soon.
     void update();
     void update(const Gfx::IntRect&);
 
+    // Repaint the widget (or an area thereof) immediately.
+    void repaint();
+    void repaint(Gfx::IntRect const&);
+
     bool is_focused() const;
     void set_focus(bool, FocusSource = FocusSource::Programmatic);
 

Userland/Libraries/LibGUI/Window.cpp

@@ -1204,4 +1204,14 @@ void Window::set_modified(bool modified)
     WindowServerConnection::the().async_set_window_modified(m_window_id, modified);
 }
 
+void Window::flush_pending_paints_immediately()
+{
+    if (!m_window_id)
+        return;
+    if (m_pending_paint_event_rects.is_empty())
+        return;
+    MultiPaintEvent paint_event(move(m_pending_paint_event_rects), size());
+    handle_multi_paint_event(paint_event);
+}
+
 }

Userland/Libraries/LibGUI/Window.h

@@ -202,6 +202,8 @@ public:
 
     Menu& add_menu(String name);
 
+    void flush_pending_paints_immediately();
+
 protected:
     Window(Core::Object* parent = nullptr);
     virtual void wm_event(WMEvent&);