QWindow: Remove "_q_foreignWinId" dynamic property

The platform plugins reading this out of the QWindow was a layering
violation, and propagates the notion that a window can shape shift
into representing a new native handle, while none of the platform
plugins support this.

A foreign QWindow is created via the factory function fromWinId(),
at which point we can pass the WId all the way to the platform
plugin as function arguments, where the platform will create a
corresponding platform-window.

The platform window can then answer the question of whether or
not it's representing a foreign window, which determines a few
behavioral changes here and there, as well as supplying the
native window handle back for QWindow::winId();

[ChangeLog][QtGui][QWindow] The "_q_foreignWinId" dynamic property
is no longer set nor read.

[ChangeLog][QtGui][QPA] The function createForeignWindow() has been
added to QPlatormIntegration and is now responsible for creating
foreign windows. The function isForeignWindow() in QPlatformWindow
has been added, and platforms should implement this to return true
for windows created by createForeignWindow().

Task-number: QTBUG-58383
Change-Id: If84142f95172f62b9377eb5d2a4d792cad36010b
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>

3 files changed, 21 insertions, 18 deletions

diff --git a/src/plugins/platforms/windows/qwindowsintegration.cpp b/src/plugins/platforms/windows/qwindowsintegration.cpp
index 5a3020387a..316f93e3ac 100644
--- a/src/plugins/platforms/windows/qwindowsintegration.cpp
+++ b/src/plugins/platforms/windows/qwindowsintegration.cpp
@@ -304,24 +304,6 @@ QPlatformWindow *QWindowsIntegration::createPlatformWindow(QWindow *window) cons
         return result;
     }
 
-    if (window->type() == Qt::ForeignWindow) {
-        const HWND hwnd = reinterpret_cast<HWND>(window->winId());
-        if (!IsWindow(hwnd)) {
-           qWarning("Windows QPA: Invalid foreign window ID %p.", hwnd);
-           return nullptr;
-        }
-        QWindowsForeignWindow *result = new QWindowsForeignWindow(window, hwnd);
-        const QRect obtainedGeometry = result->geometry();
-        QScreen *screen = Q_NULLPTR;
-        if (const QPlatformScreen *pScreen = result->screenForGeometry(obtainedGeometry))
-            screen = pScreen->screen();
-        if (screen && screen != window->screen())
-            window->setScreen(screen);
-        qCDebug(lcQpaWindows) << "Foreign window:" << window << showbase << hex
-            << result->winId() << noshowbase << dec << obtainedGeometry << screen;
-        return result;
-    }
-
     QWindowsWindowData requested;
     requested.flags = window->flags();
     requested.geometry = QHighDpi::toNativePixels(window->geometry(), window);
@@ -365,6 +347,25 @@ QPlatformWindow *QWindowsIntegration::createPlatformWindow(QWindow *window) cons
     return result;
 }
 
+QPlatformWindow *QWindowsIntegration::createForeignWindow(QWindow *window, WId nativeHandle) const
+{
+    const HWND hwnd = reinterpret_cast<HWND>(nativeHandle);
+    if (!IsWindow(hwnd)) {
+       qWarning("Windows QPA: Invalid foreign window ID %p.", hwnd);
+       return nullptr;
+    }
+    QWindowsForeignWindow *result = new QWindowsForeignWindow(window, hwnd);
+    const QRect obtainedGeometry = result->geometry();
+    QScreen *screen = Q_NULLPTR;
+    if (const QPlatformScreen *pScreen = result->screenForGeometry(obtainedGeometry))
+        screen = pScreen->screen();
+    if (screen && screen != window->screen())
+        window->setScreen(screen);
+    qCDebug(lcQpaWindows) << "Foreign window:" << window << showbase << hex
+        << result->winId() << noshowbase << dec << obtainedGeometry << screen;
+    return result;
+}
+
 // Overridden to return a QWindowsDirect2DWindow in Direct2D plugin.
 QWindowsWindow *QWindowsIntegration::createPlatformWindowHelper(QWindow *window, const QWindowsWindowData &data) const
 {
diff --git a/src/plugins/platforms/windows/qwindowsintegration.h b/src/plugins/platforms/windows/qwindowsintegration.h
index 7647b0f4a6..607203829f 100644
--- a/src/plugins/platforms/windows/qwindowsintegration.h
+++ b/src/plugins/platforms/windows/qwindowsintegration.h
@@ -73,6 +73,7 @@ public:
     bool hasCapability(QPlatformIntegration::Capability cap) const override;
 
     QPlatformWindow *createPlatformWindow(QWindow *window) const override;
+    QPlatformWindow *createForeignWindow(QWindow *window, WId nativeHandle) const override;
 #ifndef QT_NO_OPENGL
     QPlatformOpenGLContext *createPlatformOpenGLContext(QOpenGLContext *context) const override;
     QOpenGLContext::OpenGLModuleType openGLModuleType() override;
diff --git a/src/plugins/platforms/windows/qwindowswindow.h b/src/plugins/platforms/windows/qwindowswindow.h
index 10cfc18dff..e541b110a6 100644
--- a/src/plugins/platforms/windows/qwindowswindow.h
+++ b/src/plugins/platforms/windows/qwindowswindow.h
@@ -171,6 +171,7 @@ public:
     void raise() override { raise_sys(); }
     void lower() override { lower_sys(); }
     void setWindowTitle(const QString &title) override { setWindowTitle_sys(title); }
+    bool isForeignWindow() const override { return true; }
 
 protected:
     HWND handle() const override { return m_hwnd; }