LibGUI: Add GUI_HOVER_DEBUG runtime debugging flag (environment)

You can now see the outline of GUI widgets when hovering them.
For example:

    $ export GUI_HOVER_DEBUG=1
    $ FileManager

Then move the mouse around in the file manager. :^)

Meta/CMake/all_the_debug_macros.cmake

@@ -200,6 +200,8 @@ set(WSSCREEN_DEBUG ON)
 # set(DT_DEBUG ON)
 # False positive: GUI_DND_DEBUG is a flag, but passed as an envvar.
 # set(GUI_DND_DEBUG ON)
+# False positive: GUI_HOVER_DEBUG is a flag, but passed as an envvar.
+# set(GUI_HOVER_DEBUG ON)
 # False positive: GUI_FOCUS_DEBUG is a flag, but passed as an envvar.
 # set(GUI_FOCUS_DEBUG ON)
 # False positive: LOG_DEBUG is a flag, but for a bitset, not a feature.

Userland/Libraries/LibGUI/Application.cpp

@@ -81,6 +81,9 @@ Application::Application(int argc, char** argv, Core::EventLoop::MakeInspectable
     if (getenv("GUI_FOCUS_DEBUG"))
         m_focus_debugging_enabled = true;
 
+    if (getenv("GUI_HOVER_DEBUG"))
+        m_hover_debugging_enabled = true;
+
     if (getenv("GUI_DND_DEBUG"))
         m_dnd_debugging_enabled = true;
 

Userland/Libraries/LibGUI/Application.h

@@ -54,6 +54,7 @@ public:
     void set_system_palette(Core::AnonymousBuffer&);
 
     bool focus_debugging_enabled() const { return m_focus_debugging_enabled; }
+    bool hover_debugging_enabled() const { return m_hover_debugging_enabled; }
     bool dnd_debugging_enabled() const { return m_dnd_debugging_enabled; }
 
     Core::EventLoop& event_loop() { return *m_event_loop; }
@@ -102,6 +103,7 @@ private:
     WeakPtr<Window> m_active_window;
     bool m_quit_when_last_window_deleted { true };
     bool m_focus_debugging_enabled { false };
+    bool m_hover_debugging_enabled { false };
     bool m_dnd_debugging_enabled { false };
     String m_invoked_as;
     Vector<String> m_args;

Userland/Libraries/LibGUI/Widget.cpp

@@ -356,6 +356,11 @@ void Widget::handle_paint_event(PaintEvent& event)
         painter.draw_rect(rect(), Color::Cyan);
     }
 
+    if (app && app->hover_debugging_enabled() && this == window()->hovered_widget()) {
+        Painter painter(*this);
+        painter.draw_rect(rect(), Color::Red);
+    }
+
     if (is_being_inspected()) {
         Painter painter(*this);
         painter.draw_rect(rect(), Color::Magenta);

Userland/Libraries/LibGUI/Window.cpp

@@ -809,6 +809,10 @@ void Window::set_hovered_widget(Widget* widget)
 
     if (m_hovered_widget)
         Core::EventLoop::current().post_event(*m_hovered_widget, make<Event>(Event::Enter));
+
+    auto* app = Application::the();
+    if (app && app->hover_debugging_enabled())
+        update();
 }
 
 void Window::set_current_backing_store(WindowBackingStore& backing_store, bool flush_immediately)