backends/x11: Make OverlayWindow factory function private to the backend

The concept of an overlay window is specific to X11 and it's not present
on other platforms.

src/backends/x11/standalone/x11_standalone_egl_backend.cpp

@@ -16,6 +16,7 @@
 #include "surfaceitem_x11.h"
 #include "workspace.h"
 #include "x11_standalone_logging.h"
+#include "x11_standalone_overlaywindow.h"
 #include "x11_standalone_platform.h"
 
 #include <QOpenGLContext>
@@ -45,7 +46,7 @@ bool EglLayer::endFrame(const QRegion &renderedRegion, const QRegion &damagedReg
 EglBackend::EglBackend(Display *display, X11StandalonePlatform *backend)
     : EglOnXBackend(kwinApp()->x11Connection(), display, kwinApp()->x11RootWindow())
     , m_backend(backend)
-    , m_overlayWindow(backend->createOverlayWindow())
+    , m_overlayWindow(std::make_unique<OverlayWindowX11>())
     , m_layer(std::make_unique<EglLayer>(this))
 {
     // There is no any way to determine when a buffer swap completes with EGL. Fallback

src/backends/x11/standalone/x11_standalone_glx_backend.cpp

@@ -19,6 +19,7 @@
 #include "x11_standalone_glxconvenience.h"
 #include "x11_standalone_logging.h"
 #include "x11_standalone_omlsynccontrolvsyncmonitor.h"
+#include "x11_standalone_overlaywindow.h"
 #include "x11_standalone_platform.h"
 #include "x11_standalone_sgivideosyncvsyncmonitor.h"
 // kwin
@@ -121,7 +122,7 @@ bool GlxLayer::endFrame(const QRegion &renderedRegion, const QRegion &damagedReg
 
 GlxBackend::GlxBackend(Display *display, X11StandalonePlatform *backend)
     : OpenGLBackend()
-    , m_overlayWindow(kwinApp()->platform()->createOverlayWindow())
+    , m_overlayWindow(std::make_unique<OverlayWindowX11>())
     , window(None)
     , fbconfig(nullptr)
     , glxWindow(None)

src/backends/x11/standalone/x11_standalone_platform.cpp

@@ -37,7 +37,6 @@
 #include "x11_standalone_logging.h"
 #include "x11_standalone_non_composited_outline.h"
 #include "x11_standalone_output.h"
-#include "x11_standalone_overlaywindow.h"
 #include "x11_standalone_screenedges_filter.h"
 #include "xkb.h"
 
@@ -242,11 +241,6 @@ void X11StandalonePlatform::startInteractivePositionSelection(std::function<void
     m_windowSelector->start(callback);
 }
 
-std::unique_ptr<OverlayWindow> X11StandalonePlatform::createOverlayWindow()
-{
-    return std::make_unique<OverlayWindowX11>();
-}
-
 std::unique_ptr<OutlineVisual> X11StandalonePlatform::createOutline(Outline *outline)
 {
     // first try composited Outline

src/backends/x11/standalone/x11_standalone_platform.h

@@ -42,10 +42,7 @@ public:
     void createPlatformCursor(QObject *parent = nullptr) override;
     void startInteractiveWindowSelection(std::function<void(KWin::Window *)> callback, const QByteArray &cursorName = QByteArray()) override;
     void startInteractivePositionSelection(std::function<void(const QPoint &)> callback) override;
-
     PlatformCursorImage cursorImage() const override;
-
-    std::unique_ptr<OverlayWindow> createOverlayWindow() override;
     std::unique_ptr<OutlineVisual> createOutline(Outline *outline) override;
     void createEffectsHandler(Compositor *compositor, Scene *scene) override;
     QVector<CompositingType> supportedCompositors() const override;

src/core/platform.cpp

@@ -18,7 +18,6 @@
 #include "outline.h"
 #include "output.h"
 #include "outputconfiguration.h"
-#include "overlaywindow.h"
 #include "pointer_input.h"
 #include "qpainterbackend.h"
 #include "scene.h"
@@ -170,11 +169,6 @@ void Platform::startInteractivePositionSelection(std::function<void(const QPoint
     input()->startInteractivePositionSelection(callback);
 }
 
-std::unique_ptr<OverlayWindow> Platform::createOverlayWindow()
-{
-    return nullptr;
-}
-
 std::unique_ptr<OutlineVisual> Platform::createOutline(Outline *outline)
 {
     if (Compositor::compositing()) {

src/core/platform.h

@@ -28,7 +28,6 @@ class Edge;
 class Compositor;
 class DmaBufTexture;
 class InputBackend;
-class OverlayWindow;
 class OpenGLBackend;
 class Outline;
 class OutlineVisual;
@@ -171,12 +170,6 @@ public:
         m_initialOutputScale = scale;
     }
 
-    /**
-     * Creates the OverlayWindow required for X11 based compositors.
-     * Default implementation returns @c nullptr.
-     */
-    virtual std::unique_ptr<OverlayWindow> createOverlayWindow();
-
     /**
      * Creates the OutlineVisual for the given @p outline.
      * Default implementation creates an OutlineVisual suited for composited usage.