5 files changed, 31 insertions, 1 deletions
diff --git a/src/plugins/platforms/windows/qtwindowsglobal.h b/src/plugins/platforms/windows/qtwindowsglobal.h
index 573a8d07c8..3853a0afa4 100644
--- a/src/plugins/platforms/windows/qtwindowsglobal.h
+++ b/src/plugins/platforms/windows/qtwindowsglobal.h
@@ -60,6 +60,10 @@
 #  define WM_DPICHANGED 0x02E0
 #endif
 
+#ifndef WM_GETDPISCALEDSIZE
+#  define WM_GETDPISCALEDSIZE 0x02E4
+#endif
+
 // WM_POINTER support from Windows 8 onwards (WINVER >= 0x0602)
 #ifndef WM_POINTERUPDATE
 #  define WM_NCPOINTERUPDATE 0x0241
@@ -129,6 +133,7 @@ enum WindowsEventType // Simplify event types
     EnterSizeMoveEvent = WindowEventFlag + 22,
     ExitSizeMoveEvent = WindowEventFlag + 23,
     PointerActivateWindowEvent = WindowEventFlag + 24,
+    DpiScaledSizeEvent = WindowEventFlag + 25,
     MouseEvent = MouseEventFlag + 1,
     MouseWheelEvent = MouseEventFlag + 2,
     CursorEvent = MouseEventFlag + 3,
@@ -316,6 +321,8 @@ inline QtWindows::WindowsEventType windowsEventType(UINT message, WPARAM wParamI
         return HIWORD(wParamIn) ? QtWindows::AcceleratorCommandEvent : QtWindows::MenuCommandEvent;
     case WM_DPICHANGED:
         return QtWindows::DpiChangedEvent;
+    case WM_GETDPISCALEDSIZE:
+        return QtWindows::DpiScaledSizeEvent;
     case WM_ENTERSIZEMOVE:
         return QtWindows::EnterSizeMoveEvent;
     case WM_EXITSIZEMOVE:
diff --git a/src/plugins/platforms/windows/qwindowscontext.cpp b/src/plugins/platforms/windows/qwindowscontext.cpp
index 91fb80dc28..1c75f7f7d2 100644
--- a/src/plugins/platforms/windows/qwindowscontext.cpp
+++ b/src/plugins/platforms/windows/qwindowscontext.cpp
@@ -1453,6 +1453,9 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
         return true;
 #endif
     }   break;
+    case QtWindows::DpiScaledSizeEvent:
+        platformWindow->handleDpiScaledSize(wParam, lParam, result);
+        return true;
     case QtWindows::DpiChangedEvent:
         platformWindow->handleDpiChanged(hwnd, wParam, lParam);
         return true;
diff --git a/src/plugins/platforms/windows/qwindowsscreen.h b/src/plugins/platforms/windows/qwindowsscreen.h
index 2052648933..319e998c80 100644
--- a/src/plugins/platforms/windows/qwindowsscreen.h
+++ b/src/plugins/platforms/windows/qwindowsscreen.h
@@ -86,7 +86,7 @@ public:
     QImage::Format format() const override { return m_data.format; }
     QSizeF physicalSize() const override { return m_data.physicalSizeMM; }
     QDpi logicalDpi() const override { return m_data.dpi; }
-    QDpi logicalBaseDpi() const override { return QDpi(96, 96); }
+    QDpi logicalBaseDpi() const override { return QDpi(baseDpi, baseDpi); }
     qreal devicePixelRatio() const override { return 1.0; }
     qreal refreshRate() const override { return m_data.refreshRateHz; }
     QString name() const override { return m_data.name; }
@@ -115,6 +115,7 @@ public:
     const QWindowsScreenData &data() const  { return m_data; }
 
     static QRect virtualGeometry(const QPlatformScreen *screen);
+    static inline int baseDpi = 96;
 
 private:
     QWindowsScreenData m_data;
diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index 3c64ae1a35..2934af5502 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/src/plugins/platforms/windows/qwindowswindow.cpp
@@ -1833,6 +1833,24 @@ void QWindowsWindow::handleCompositionSettingsChanged()
     }
 }
 
+void QWindowsWindow::handleDpiScaledSize(WPARAM wParam, LPARAM lParam, LRESULT *result)
+{
+    // We want to keep QWindow's device independent size constant across the
+    // DPI change. To accomplish this, scale QPlatformWindow's native size
+    // by the change of DPI (e.g. 120 -> 144 = 1.2), also taking any scale
+    // factor rounding into account. The win32 window size includes the margins;
+    // add the margins for the new DPI to the window size.
+    const int dpi = int(wParam);
+    const qreal scale = QHighDpiScaling::roundScaleFactor(qreal(dpi) / QWindowsScreen::baseDpi) /
+                        QHighDpiScaling::roundScaleFactor(qreal(savedDpi()) / QWindowsScreen::baseDpi);
+    const QMargins margins = QWindowsGeometryHint::frame(style(), exStyle(), dpi);
+    const QSize windowSize = (geometry().size() * scale).grownBy(margins);
+    SIZE *size = reinterpret_cast<SIZE *>(lParam);
+    size->cx = windowSize.width();
+    size->cy = windowSize.height();
+    *result = true; // Inform Windows that we've set a size
+}
+
 void QWindowsWindow::handleDpiChanged(HWND hwnd, WPARAM wParam, LPARAM lParam)
 {
     const UINT dpi = HIWORD(wParam);
diff --git a/src/plugins/platforms/windows/qwindowswindow.h b/src/plugins/platforms/windows/qwindowswindow.h
index 14c5f47853..c02f9a7fe7 100644
--- a/src/plugins/platforms/windows/qwindowswindow.h
+++ b/src/plugins/platforms/windows/qwindowswindow.h
@@ -318,6 +318,7 @@ public:
     void handleResized(int wParam);
     void handleHidden();
     void handleCompositionSettingsChanged();
+    void handleDpiScaledSize(WPARAM wParam, LPARAM lParam, LRESULT *result);
     void handleDpiChanged(HWND hwnd, WPARAM wParam, LPARAM lParam);
 
     static void displayChanged();