Fix programatic geometry change to different DPI

The event flow for programatic window geometry change
(e.g. from MoveWindow()) differs from user-interactive
geometry change: We still get WM_DPICHANGED, but this
event is not preceded by WM_GETDPISCALEDSIZE, so we don’t
get to override the window size.

However, Qt has already scaled the window size for the
new DPI in this case (the scaled size is provided to
QWindowsWindow::setGeometry()), so we can omit making
second native set-geometry call.

Change-Id: Ia7d42d7fee49adf757e7fe75d77f1731405ad519
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 81707951fe251d9c9074a5b8ca6211900a23d235)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 27 insertions, 8 deletions

diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index bbe19e4fcc..f167847a59 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/src/plugins/platforms/windows/qwindowswindow.cpp
@@ -1345,6 +1345,7 @@ void QWindowCreationContext::applyToMinMaxInfo(MINMAXINFO *mmi) const
 const char *QWindowsWindow::embeddedNativeParentHandleProperty = "_q_embedded_native_parent_handle";
 const char *QWindowsWindow::hasBorderInFullScreenProperty = "_q_has_border_in_fullscreen";
 bool QWindowsWindow::m_borderInFullScreenDefault = false;
+bool QWindowsWindow::m_inSetgeometry = false;
 
 QWindowsWindow::QWindowsWindow(QWindow *aWindow, const QWindowsWindowData &data) :
     QWindowsBaseWindow(aWindow),
@@ -1842,14 +1843,29 @@ void QWindowsWindow::handleDpiChanged(HWND hwnd, WPARAM wParam, LPARAM lParam)
     // Send screen change first, so that the new sceen is set during any following resize
     checkForScreenChanged(QWindowsWindow::FromDpiChange);
 
-    // Apply the suggested window geometry to the native window. This will make
-    // sure the window tracks the mouse cursor during screen change, and also
-    // that the window size is scaled according to the DPI change.
-    updateFullFrameMargins();
-    const auto prcNewWindow = reinterpret_cast<RECT *>(lParam);
-    SetWindowPos(hwnd, nullptr, prcNewWindow->left, prcNewWindow->top,
-                 prcNewWindow->right - prcNewWindow->left,
-                 prcNewWindow->bottom - prcNewWindow->top, SWP_NOZORDER | SWP_NOACTIVATE);
+    // We get WM_DPICHANGED in one of two situations:
+    //
+    // 1. The DPI change is a "spontaneous" DPI change as a result of e.g.
+    // the user dragging the window to a new screen. In this case Windows
+    // first sends WM_GETDPISCALEDSIZE, where we set the new window size,
+    // followed by this event where we apply the suggested window geometry
+    // to the native window. This will make sure the window tracks the mouse
+    // cursor during screen change, and also that the window size is scaled
+    // according to the DPI change.
+    //
+    // 2. The DPI change is a result of a setGeometry() call. In this case
+    // Qt has already scaled the window size for the new DPI. Further, Windows
+    // does not call WM_GETDPISCALEDSIZE, and also applies its own scaling
+    // to the already scaled window size. Since there is no need to set the
+    // window geometry again, and the provided geometry is incorrect, we omit
+    // making the SetWindowPos() call.
+    if (!m_inSetgeometry) {
+        updateFullFrameMargins();
+        const auto prcNewWindow = reinterpret_cast<RECT *>(lParam);
+        SetWindowPos(hwnd, nullptr, prcNewWindow->left, prcNewWindow->top,
+                     prcNewWindow->right - prcNewWindow->left,
+                     prcNewWindow->bottom - prcNewWindow->top, SWP_NOZORDER | SWP_NOACTIVATE);
+    }
 
     // Scale child QPlatformWindow size. Windows sends WM_DPICHANGE to top-level windows only.
     for (QWindow *childWindow : window()->findChildren<QWindow *>()) {
@@ -1930,6 +1946,8 @@ static QString msgUnableToSetGeometry(const QWindowsWindow *platformWindow,
 
 void QWindowsWindow::setGeometry(const QRect &rectIn)
 {
+    QBoolBlocker b(m_inSetgeometry);
+
     QRect rect = rectIn;
     // This means it is a call from QWindow::setFramePosition() and
     // the coordinates include the frame (size is still the contents rectangle).
diff --git a/src/plugins/platforms/windows/qwindowswindow.h b/src/plugins/platforms/windows/qwindowswindow.h
index 14c5f47853..205a02a30d 100644
--- a/src/plugins/platforms/windows/qwindowswindow.h
+++ b/src/plugins/platforms/windows/qwindowswindow.h
@@ -417,6 +417,7 @@ private:
     VkSurfaceKHR m_vkSurface = VK_NULL_HANDLE;
 #endif
     static bool m_borderInFullScreenDefault;
+    static bool m_inSetgeometry;
 };
 
 #ifndef QT_NO_DEBUG_STREAM