[UI] Set fullscreen on double left mouse click

src/xenia/app/emulator_window.h

@@ -38,6 +38,8 @@ struct RecentTitleEntry {
 
 class EmulatorWindow {
  public:
+  using steady_clock = std::chrono::steady_clock;  // stdlib steady clock
+
   enum : size_t {
     // The UI is on top of the game and is open in special cases, so
     // lowest-priority.
@@ -57,7 +59,17 @@ class EmulatorWindow {
   std::unique_ptr<xe::threading::Thread> Gamepad_HotKeys_Listener;
 
   int32_t selected_title_index = -1;
-  
+
+  static constexpr int64_t diff_in_ms(
+    const steady_clock::time_point t1,
+    const steady_clock::time_point t2) noexcept {
+    using ms = std::chrono::milliseconds;
+    return std::chrono::duration_cast<ms>(t1 - t2).count();
+  }
+
+  steady_clock::time_point last_mouse_up = steady_clock::now();
+  steady_clock::time_point last_mouse_down = steady_clock::now();
+
   Emulator* emulator() const { return emulator_; }
   ui::WindowedAppContext& app_context() const { return app_context_; }
   ui::Window* window() const { return window_.get(); }
@@ -131,6 +143,9 @@ class EmulatorWindow {
 
     void OnKeyDown(ui::KeyEvent& e) override;
 
+    void OnMouseDown(ui::MouseEvent& e) override;
+    void OnMouseUp(ui::MouseEvent& e) override;
+
    private:
     EmulatorWindow& emulator_window_;
   };
@@ -184,7 +199,10 @@ class EmulatorWindow {
   void ApplyDisplayConfigForCvars();
 
   void OnKeyDown(ui::KeyEvent& e);
+  void OnMouseDown(const ui::MouseEvent& e);
+  void ToggleFullscreenOnDoubleClick();
   void FileDrop(const std::filesystem::path& filename);
+  void OnMouseUp(const ui::MouseEvent& e);
   void FileOpen();
   void FileClose();
   void InstallContent();