Make resizeMaximizedWindows() preserve native geometry

Previously, resizeMaximizedWindows() would use the device
independent screen size as the source of truth when
setting window sizes. However this size may have been
rounded, which means that e.g. a fullscreen window may
fail to cover the entire screen.

Instead, use the native screen size as the true screen
size. Set QPlatformWindow geometry, and let the
platform update QWindow geometry via geometry change
events.

Fixes: QTBUG-87334
Change-Id: If6e4852dea46ab03c83e469808c0047bc933ee47
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 1e85dfacf3479ebbe5a4502895b8b8d93fbe1477)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 13 insertions, 6 deletions

diff --git a/src/gui/kernel/qplatformscreen.cpp b/src/gui/kernel/qplatformscreen.cpp
index f1fe942b9c..0649139455 100644
--- a/src/gui/kernel/qplatformscreen.cpp
+++ b/src/gui/kernel/qplatformscreen.cpp
@@ -47,6 +47,7 @@
 #include <QtGui/qscreen.h>
 #include <QtGui/qwindow.h>
 #include <private/qhighdpiscaling_p.h>
+#include <private/qwindow_p.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -353,11 +354,12 @@ QPlatformCursor *QPlatformScreen::cursor() const
 */
 void QPlatformScreen::resizeMaximizedWindows()
 {
-    // 'screen()' still has the old geometry info while 'this' has the new geometry info
+    // 'screen()' still has the old geometry (in device independent pixels),
+    // while 'this' has the new geometry (in native pixels)
     const QRect oldGeometry = screen()->geometry();
     const QRect oldAvailableGeometry = screen()->availableGeometry();
-    const QRect newGeometry = deviceIndependentGeometry();
-    const QRect newAvailableGeometry = QHighDpi::fromNative(availableGeometry(), QHighDpiScaling::factor(this), newGeometry.topLeft());
+    const QRect newNativeGeometry = this->geometry();
+    const QRect newNativeAvailableGeometry = this->availableGeometry();
 
     const bool supportsMaximizeUsingFullscreen = QGuiApplicationPrivate::platformIntegration()->hasCapability(QPlatformIntegration::MaximizeUsingFullscreenGeometry);
 
@@ -366,14 +368,19 @@ void QPlatformScreen::resizeMaximizedWindows()
         if (!w->handle())
             continue;
 
+        // Set QPlatformWindow size in native pixels, and let the platform's geometry
+        // change signals update the QWindow geomeyry. This way we make sure that the
+        // platform window geometry covers the entire (available) platform screen geometry,
+        // also when fractional DPRs introduce rounding errors in the device independent
+        // QWindow and QScreen sizes.
         if (supportsMaximizeUsingFullscreen
                 && w->windowState() & Qt::WindowMaximized
                 && w->flags() & Qt::MaximizeUsingFullscreenGeometryHint) {
-            w->setGeometry(newGeometry);
+            w->handle()->setGeometry(newNativeGeometry);
         } else if (w->windowState() & Qt::WindowMaximized || w->geometry() == oldAvailableGeometry) {
-            w->setGeometry(newAvailableGeometry);
+            w->handle()->setGeometry(newNativeAvailableGeometry);
         } else if (w->windowState() & Qt::WindowFullScreen || w->geometry() == oldGeometry) {
-            w->setGeometry(newGeometry);
+            w->handle()->setGeometry(newNativeGeometry);
         }
     }
 }