iOS plugin: Make sure window is of type QUIWindow when determining the color scheme

In the iOS theme, we determine the color scheme based on the the last
UIWindow of the application, but listen to trait changes in the
QUIWindow class. However, the last window of the application is not
always a QUIWindow. Sometimes it can be a temporary UIWindow created
by the system for transitioning or other effects. These kind of windows
do not always follow the appearance of the app and the main window
(QUIWindow), so we were sometimes ending up with the wrong color scheme being reported.

This was happening when the app was put into background for example,
which causes the traitCollectionDidChange method to be called and query
the userInterfaceStyle of the last window to determine if the color scheme was changed. The queried window would sometimes end up being of type UITextEffectsWindow, etc. and report the wrong appearance.
To fix, always make sure to get the appearance from a QUIWindow.

Fixes: QTBUG-114571
Fixes: QTBUG-113169
Fixes: QTBUG-114191
Pick-to: 6.5 6.6 6.5.2
Change-Id: Ic0b29c02c8e8100996d5cd31b37e6a5b839f5fb1
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 10 insertions, 6 deletions

diff --git a/src/plugins/platforms/ios/qiostheme.mm b/src/plugins/platforms/ios/qiostheme.mm
index d1356ab8f7..fad4a8d581 100644
--- a/src/plugins/platforms/ios/qiostheme.mm
+++ b/src/plugins/platforms/ios/qiostheme.mm
@@ -23,6 +23,7 @@
 #include "qiosmessagedialog.h"
 #include "qioscolordialog.h"
 #include "qiosfontdialog.h"
+#include "qiosscreen.h"
 #endif
 
 QT_BEGIN_NAMESPACE
@@ -144,14 +145,17 @@ QVariant QIOSTheme::themeHint(ThemeHint hint) const
 
 Qt::ColorScheme QIOSTheme::colorScheme() const
 {
-    UIUserInterfaceStyle appearance = UIUserInterfaceStyleUnspecified;
-    // Set the appearance based on the UIWindow
+    // Set the appearance based on the QUIWindow
     // Fallback to the UIScreen if no window is created yet
-    if (UIWindow *window = qt_apple_sharedApplication().windows.lastObject) {
-        appearance = window.traitCollection.userInterfaceStyle;
-    } else {
-        appearance = UIScreen.mainScreen.traitCollection.userInterfaceStyle;
+    UIUserInterfaceStyle appearance = UIScreen.mainScreen.traitCollection.userInterfaceStyle;
+    NSArray<UIWindow *> *windows = qt_apple_sharedApplication().windows;
+    for (UIWindow *window in windows) {
+        if ([window isKindOfClass:[QUIWindow class]]) {
+            appearance = static_cast<QUIWindow*>(window).traitCollection.userInterfaceStyle;
+            break;
+        }
     }
+
     return appearance == UIUserInterfaceStyleDark
                        ? Qt::ColorScheme::Dark
                        : Qt::ColorScheme::Light;