Fix window auto-focus on Linux

Source/Engine/Platform/Linux/LinuxPlatform.cpp

@@ -1691,6 +1691,15 @@ void LinuxPlatform::Tick()
 					}
 				}
 			break;
+			case MapNotify:
+				// Auto-focus shown windows
+				window = WindowsManager::GetByNativePtr((void*)event.xmap.window);
+				if (window && window->_focusOnMapped)
+				{
+					window->_focusOnMapped = false;
+					window->Focus();
+				}
+			break;
 			case FocusIn:
 				// Update input context focus
 				X11::XSetICFocus(IC);

Source/Engine/Platform/Linux/LinuxWindow.cpp

@@ -242,10 +242,7 @@ void LinuxWindow::Show()
 		LINUX_WINDOW_PROLOG;
 		X11::XMapWindow(display, window);
 		X11::XFlush(display);
-		if (_settings.AllowInput && _settings.ActivateWhenFirstShown)
-		{
-			Focus();
-		}
+		_focusOnMapped = _settings.AllowInput && _settings.ActivateWhenFirstShown;
 
 		// Base
 		WindowBase::Show();
@@ -306,6 +303,11 @@ bool LinuxWindow::IsClosed() const
 	return _isClosing;
 }
 
+bool LinuxWindow::IsForegroundWindow() const
+{
+	return _focused || _focusOnMapped;
+}
+
 void LinuxWindow::SetClientBounds(const Rectangle& clientArea)
 {
 	int32 x = Math::TruncToInt(clientArea.GetX());
@@ -673,7 +675,14 @@ void LinuxWindow::SetOpacity(const float opacity)
 
 void LinuxWindow::Focus()
 {
-	// TODO: impl this
+	if (_focused || !IsWindowMapped())
+		return;
+
+	LINUX_WINDOW_PROLOG;
+
+	X11::XSetInputFocus(display, window, RevertToPointerRoot, CurrentTime);
+	X11::XFlush(display);
+	X11::XSync(display, 0);
 }
 
 void LinuxWindow::SetTitle(const StringView& title)

Source/Engine/Platform/Linux/LinuxWindow.h

@@ -20,7 +20,7 @@ public:
 
 private:
 
-	bool _resizeDisabled;
+	bool _resizeDisabled, _focusOnMapped = false;
 	float _opacity = 1.0f;
 	HandleType _window;
 
@@ -74,6 +74,7 @@ public:
 	void Maximize() override;
 	void Restore() override;
 	bool IsClosed() const override;
+    bool IsForegroundWindow() const override;
 	void BringToFront(bool force = false) override;
 	void SetClientBounds(const Rectangle& clientArea) override;
 	void SetPosition(const Vector2& position) override;