Windows: better handling of darkmode support

43ef220 turned dark mode support on for
both styling and window frames. However, the default palette and style
support in Qt is too incomplete, resulting in unreadable UIs when using
certain styles (e.g. fusion). Also the vista style is not supporting
dark mode.

If we don't turn on dark style support, then dark frame support doesn't
look good either. However, many application developers have implement a
dark theme themselves, and we should have a dark frame for those
applications.

So partially revert 43ef220 so that
dark style support is disabled by default, and leave dark frame support
on. However, only activate dark frames if the palette is dark, i.e. if
the window background color in the default palette is darker than the
text color (or if DarkModeStyle is explicitly turned on by running the
application with -platform windows:darkmode=2).

This way, dark-themed applications get a dark frame on dark Windows, and
a light frame on light Windows; and light-themed applications (including
default Qt applications) get a light frame all the time.

Fixes: QTBUG-72028
Pick-to: 6.4
Change-Id: I61f1b1e43b2a4ba69848d5d8bec921c0790fe511
Reviewed-by: Marius Kittler <[email protected]>
Reviewed-by: Oliver Wolff <[email protected]>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1103,14 +1103,14 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
             if (darkMode != QWindowsContextPrivate::m_darkMode) {
                 QWindowsContextPrivate::m_darkMode = darkMode;
                 auto integration = QWindowsIntegration::instance();
-                if (integration->darkModeHandling().testFlag(QWindowsApplication::DarkModeWindowFrames)) {
-                    for (QWindowsWindow *w : d->m_windows)
-                        w->setDarkBorder(QWindowsContextPrivate::m_darkMode);
-                }
                 if (integration->darkModeHandling().testFlag(QWindowsApplication::DarkModeStyle)) {
                     QWindowsTheme::instance()->refresh();
                     QWindowSystemInterface::handleThemeChange();
                 }
+                if (integration->darkModeHandling().testFlag(QWindowsApplication::DarkModeWindowFrames)) {
+                    for (QWindowsWindow *w : d->m_windows)
+                        w->setDarkBorder(QWindowsContextPrivate::m_darkMode);
+                }
             }
         }
         return d->m_screenManager.handleScreenChanges();

src/plugins/platforms/windows/qwindowsintegration.cpp

@@ -212,8 +212,7 @@ void QWindowsIntegrationPrivate::parseOptions(QWindowsIntegration *q, const QStr
     QtWindows::ProcessDpiAwareness dpiAwareness = QtWindows::ProcessPerMonitorV2DpiAware;
 
     int tabletAbsoluteRange = -1;
-    DarkModeHandling darkModeHandling = DarkModeHandlingFlag::DarkModeWindowFrames
-                                      | DarkModeHandlingFlag::DarkModeStyle;
+    DarkModeHandling darkModeHandling = DarkModeHandlingFlag::DarkModeWindowFrames;
     m_options = ::parseOptions(paramList, &tabletAbsoluteRange, &dpiAwareness, &darkModeHandling);
     q->setDarkModeHandling(darkModeHandling);
     QWindowsFontDatabase::setFontOptions(m_options);

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -848,7 +848,15 @@ void WindowCreationData::fromWindow(const QWindow *w, const Qt::WindowFlags flag
 
 static inline bool shouldApplyDarkFrame(const QWindow *w)
 {
-    return w->isTopLevel() && !w->flags().testFlag(Qt::FramelessWindowHint);
+    if (!w->isTopLevel() || w->flags().testFlag(Qt::FramelessWindowHint))
+        return false;
+    if (QWindowsIntegration::instance()->darkModeHandling().testFlag(QWindowsApplication::DarkModeStyle))
+        return true;
+    // if the application supports a dark border, and the palette is dark (window background color
+    // is darker than the text), then turn dark-border support on, otherwise use a light border.
+    const QPalette defaultPalette;
+    return defaultPalette.color(QPalette::WindowText).lightness()
+         > defaultPalette.color(QPalette::Window).lightness();
 }
 
 QWindowsWindowData