Windows QPA: Correctly respond to WM_ERASEBKGND

According to Microsoft Docs [1], applications should
return non-zero in response to WM_ERASEBKGND if it
processes the message and erases the background and
that's indeed the case for Qt.

Although I can't see any visual difference, this patch
obeys the official documentation at least.

[1] https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-erasebkgnd

Change-Id: I8aa0bfb25259013bfc2ca4074f05a97c7865159c
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
(cherry picked from commit 5d129426765871912b4f0b4ba77eef4b338d00a3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

3 files changed, 6 insertions, 4 deletions

diff --git a/src/plugins/platforms/windows/qwindowscontext.cpp b/src/plugins/platforms/windows/qwindowscontext.cpp
index 4ec767afa4..7cb00d4a0a 100644
--- a/src/plugins/platforms/windows/qwindowscontext.cpp
+++ b/src/plugins/platforms/windows/qwindowscontext.cpp
@@ -1268,7 +1268,7 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
     case QtWindows::GeometryChangingEvent:
         return platformWindow->QWindowsWindow::handleGeometryChanging(&msg);
     case QtWindows::ExposeEvent:
-        return platformWindow->handleWmPaint(hwnd, message, wParam, lParam);
+        return platformWindow->handleWmPaint(hwnd, message, wParam, lParam, result);
     case QtWindows::NonClientMouseEvent:
         if ((d->m_systemInfo & QWindowsContext::SI_SupportsPointer) && platformWindow->frameStrutEventsEnabled())
             return sessionManagerInteractionBlocked() || d->m_pointerHandler.translateMouseEvent(platformWindow->window(), hwnd, et, msg, result);
diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index b100015f62..d77c1ad8f0 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/src/plugins/platforms/windows/qwindowswindow.cpp
@@ -2157,10 +2157,12 @@ static inline bool isSoftwareGl()
 }
 
 bool QWindowsWindow::handleWmPaint(HWND hwnd, UINT message,
-                                         WPARAM, LPARAM)
+                                         WPARAM, LPARAM, LRESULT *result)
 {
-    if (message == WM_ERASEBKGND) // Backing store - ignored.
+    if (message == WM_ERASEBKGND) { // Backing store - ignored.
+        *result = 1;
         return true;
+    }
     // QTBUG-75455: Suppress WM_PAINT sent to invisible windows when setting WS_EX_LAYERED
     if (!window()->isVisible() && (GetWindowLong(hwnd, GWL_EXSTYLE) & WS_EX_LAYERED) != 0)
         return false;
diff --git a/src/plugins/platforms/windows/qwindowswindow.h b/src/plugins/platforms/windows/qwindowswindow.h
index 7123afa11a..5e8478ca9c 100644
--- a/src/plugins/platforms/windows/qwindowswindow.h
+++ b/src/plugins/platforms/windows/qwindowswindow.h
@@ -313,7 +313,7 @@ public:
     void setStyle(unsigned s) const;
     void setExStyle(unsigned s) const;
 
-    bool handleWmPaint(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);
+    bool handleWmPaint(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam, LRESULT *result);
 
     void handleMoved();
     void handleResized(int wParam);