Track current window DPI

The WM_DPICHANGED event gives us the new DPI, but we
also need the current DPI in order to determine the
scale factor corresponding to the DPI change.

Pick-to: 6.2
Change-Id: Ia61388415f57aa739397d3125b8751952e8fd392
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -218,6 +218,7 @@ void QWindowsUser32DLL::init()
         getWindowDpiAwarenessContext = (GetWindowDpiAwarenessContext)library.resolve("GetWindowDpiAwarenessContext");
         getAwarenessFromDpiAwarenessContext = (GetAwarenessFromDpiAwarenessContext)library.resolve("GetAwarenessFromDpiAwarenessContext");
         systemParametersInfoForDpi = (SystemParametersInfoForDpi)library.resolve("SystemParametersInfoForDpi");
+        getDpiForWindow = (GetDpiForWindow)library.resolve("GetDpiForWindow");
     }
 }
 
@@ -1474,6 +1475,10 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
 #endif
     }   break;
     case QtWindows::DpiChangedEvent: {
+
+        const UINT dpi = HIWORD(wParam);
+        platformWindow->setSavedDpi(dpi);
+
         // Try to apply the suggested size first and then notify ScreenChanged
         // so that the resize event sent from QGuiApplication incorporates it
         // WM_DPICHANGED is sent with a size that avoids resize loops (by

src/plugins/platforms/windows/qwindowscontext.h

@@ -108,6 +108,7 @@ struct QWindowsUser32DLL
     typedef int  (WINAPI *GetWindowDpiAwarenessContext)(HWND);
     typedef int  (WINAPI *GetAwarenessFromDpiAwarenessContext)(int);
     typedef BOOL (WINAPI *SystemParametersInfoForDpi)(UINT, UINT, PVOID, UINT, UINT);
+    typedef int  (WINAPI *GetDpiForWindow)(HWND);
 
     // Windows pointer functions (Windows 8 or later).
     EnableMouseInPointer enableMouseInPointer = nullptr;
@@ -124,6 +125,9 @@ struct QWindowsUser32DLL
     // Windows Vista onwards
     SetProcessDPIAware setProcessDPIAware = nullptr;
 
+    // Windows 10 version 1607 onwards
+    GetDpiForWindow getDpiForWindow = nullptr;
+
     // Windows 10 version 1703 onwards
     SetProcessDpiAwarenessContext setProcessDpiAwarenessContext = nullptr;
 

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1419,6 +1419,8 @@ void QWindowsWindow::initialize()
         if (obtainedScreen && screen() != obtainedScreen)
             QWindowSystemInterface::handleWindowScreenChanged<QWindowSystemInterface::SynchronousDelivery>(w, obtainedScreen->screen());
     }
+    QWindowsWindow::setSavedDpi(QWindowsContext::user32dll.getDpiForWindow ?
+        QWindowsContext::user32dll.getDpiForWindow(handle()) : 96);
 }
 
 QSurfaceFormat QWindowsWindow::format() const

src/plugins/platforms/windows/qwindowswindow.h

@@ -372,6 +372,9 @@ public:
     static const char *embeddedNativeParentHandleProperty;
     static const char *hasBorderInFullScreenProperty;
 
+    void setSavedDpi(int dpi) { m_savedDpi = dpi; }
+    int savedDpi() const { return m_savedDpi; }
+
 private:
     inline void show_sys() const;
     inline QWindowsWindowData setWindowFlags_sys(Qt::WindowFlags wt, unsigned flags = 0) const;
@@ -405,6 +408,7 @@ private:
     HICON m_iconSmall = nullptr;
     HICON m_iconBig = nullptr;
     void *m_surface = nullptr;
+    int m_savedDpi = 96;
 
     static bool m_screenForGLInitialized;