Windows: fix fullscreen OpenGL window issues with an opt-in function

Windows automatically disables DWM for opengl windows that have the
exact dimensions of the primary screen. This causes numerous issues,
such as menus and popups not showing up, and alt+tab misbehavior.

Adding a one pixel border via WS_BORDER solves all of these issues.
This is done by a QWindowsWindowFunctions to make it opt-in as turning
it on can cause an unwanted change in the look of the window so it is
up to the user to decide if they want this.

[ChangeLog][Platform Specific Changes][Windows] Add a function to
QWindowsWindowFunctions to enable working around a limitation with
showing other top level windows when showing a fullscreen OpenGL based
window.

Task-number: QTBUG-41309
Task-number: QTBUG-41883
Task-number: QTBUG-42410
Change-Id: I8c5c785f5024737cd034b2b703671632a8102700
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/platformheaders/windowsfunctions/qwindowswindowfunctions.h

@@ -60,6 +60,15 @@ public:
         if (func)
             func(window, type);
     }
+
+    typedef void (*SetHasBorderInFullScreen)(QWindow *window, bool border);
+    static const QByteArray setHasBorderInFullScreenIdentifier() { return QByteArrayLiteral("WindowsSetHasBorderInFullScreen"); }
+    static void setHasBorderInFullScreen(QWindow *window, bool border)
+    {
+        SetHasBorderInFullScreen func = reinterpret_cast<SetHasBorderInFullScreen>(QGuiApplication::platformFunction(setHasBorderInFullScreenIdentifier()));
+        if (func)
+            func(window, border);
+    }
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS(QWindowsWindowFunctions::TouchWindowTouchTypes)

src/platformheaders/windowsfunctions/qwindowswindowfunctions.qdoc

@@ -69,3 +69,29 @@
     This is a convenience function that can be used directly instead of resolving the function pointer.
     \a window and \a type will be relayed to the function retrieved by QGuiApplication
 */
+
+/*!
+    \typedef QWindowsWindowFunctions::SetHasBorderInFullScreen
+
+    This is the typedef for the function returned by QGuiApplication::platformFunction when passed setHasBorderInFullScreenIdentifier.
+*/
+
+/*!
+    \fn QByteArray QWindowsWindowFunctions::setHasBorderInFullScreenIdentifier()
+    \since 5.6
+
+    This function returns the bytearray that can be used to query
+    QGuiApplication::platformFunction to retrieve the SetHasBorderInFullScreen function.
+*/
+
+/*!
+    \fn void QWindowsWindowFunctions::setHasBorderInFullScreen(QWindow *window, bool border)
+    \since 5.6
+
+    This is a convenience function that can be used directly instead of resolving the function pointer.
+    \a window and \a border will be relayed to the function retrieved by QGuiApplication. When \a border
+    is true then it will enable the WS_BORDER flag in full screen mode to enable other top level windows
+    inside the application to appear on top when required.
+
+    See also \l [QtDoc] {Fullscreen OpenGL Based Windows}
+*/

src/plugins/platforms/windows/qwindowsnativeinterface.cpp

@@ -234,6 +234,8 @@ QFunctionPointer QWindowsNativeInterface::platformFunction(const QByteArray &fun
 {
     if (function == QWindowsWindowFunctions::setTouchWindowTouchTypeIdentifier())
         return QFunctionPointer(QWindowsWindow::setTouchWindowTouchTypeStatic);
+    else if (function == QWindowsWindowFunctions::setHasBorderInFullScreenIdentifier())
+        return QFunctionPointer(QWindowsWindow::setHasBorderInFullScreenStatic);
     return Q_NULLPTR;
 }
 

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1735,6 +1735,8 @@ void QWindowsWindow::setWindowState_sys(Qt::WindowState newState)
                 newStyle |= WS_SYSMENU;
             if (visible)
                 newStyle |= WS_VISIBLE;
+            if (testFlag(HasBorderInFullScreen))
+                newStyle |= WS_BORDER;
             setStyle(newStyle);
             // Use geometry of QWindow::screen() within creation or the virtual screen the
             // window is in (QTBUG-31166, QTBUG-30724).
@@ -2371,4 +2373,19 @@ void QWindowsWindow::aboutToMakeCurrent()
 #endif
 }
 
+void QWindowsWindow::setHasBorderInFullScreenStatic(QWindow *window, bool border)
+{
+    if (!window->handle())
+        return;
+    static_cast<QWindowsWindow *>(window->handle())->setHasBorderInFullScreen(border);
+}
+
+void QWindowsWindow::setHasBorderInFullScreen(bool border)
+{
+    if (border)
+        setFlag(HasBorderInFullScreen);
+    else
+        clearFlag(HasBorderInFullScreen);
+}
+
 QT_END_NAMESPACE

src/plugins/platforms/windows/qwindowswindow.h

@@ -137,7 +137,8 @@ public:
         WithinMaximize = 0x40000,
         MaximizeToFullScreen = 0x80000,
         InputMethodDisabled = 0x100000,
-        Compositing = 0x200000
+        Compositing = 0x200000,
+        HasBorderInFullScreen = 0x400000
     };
 
     QWindowsWindow(QWindow *window, const QWindowsWindowData &data);
@@ -251,7 +252,8 @@ public:
 
     static void setTouchWindowTouchTypeStatic(QWindow *window, QWindowsWindowFunctions::TouchWindowTouchTypes touchTypes);
     void registerTouchWindow(QWindowsWindowFunctions::TouchWindowTouchTypes touchTypes = QWindowsWindowFunctions::NormalTouch);
-
+    static void setHasBorderInFullScreenStatic(QWindow *window, bool border);
+    void setHasBorderInFullScreen(bool border);
 private:
     inline void show_sys() const;
     inline void hide_sys() const;