Remove alpha component of system colors

When using system colors it may happen (on windows 11 in particular)
that the background color is actually translucid. This is not
something we are prepared to handle and it creates ghosts on screen.

The fix is easy: the alpha channel of the colors is set to 255.

Fixes bug #13084.

(cherry picked from commit 095bb18331)

src/frontends/qt/ColorCache.cpp

@@ -86,7 +86,9 @@ QColor ColorCache::get(Color const & color, bool syscolors) const
 		QPalette::ColorRole const cr = role(color.baseColor);
 		if (syscolors && cr != QPalette::NoRole) {
 			static QColor const white = Qt::white;
-			QColor const c = pal_.brush(QPalette::Active, cr).color();
+			QColor c = pal_.brush(QPalette::Active, cr).color();
+			// Change to fully opaque color
+			c.setAlpha(255);
 			if (cr == QPalette::Base && c == white)
 				return lcolors_[color.baseColor];
 			else

status.24x

@@ -67,6 +67,8 @@ What's new
 - Enable OK/Apply button in tabular dialog when "Additional Space" combo has
   been set to different value.
 
+- Fix ghost image on Windows with system colors (bug 13084).
+
 
 * INTERNALS