Some applications have black dock widgets

[10110111]

E.g. CI. When this happens, we have repeating Qt warnings in the terminal, like this:

QXcbConnection: XCB error: 8 (BadMatch), sequence: 1758, resource id: 186646640, major code: 130 (Unknown), minor code: 3

This can be worked around by this change:

diff --git a/style/oxygenstyle.cpp b/style/oxygenstyle.cpp
index 243e58a..8187567 100644
--- a/style/oxygenstyle.cpp
+++ b/style/oxygenstyle.cpp
@@ -427,7 +427,7 @@ namespace Oxygen
         } else if( qobject_cast<QDockWidget*>( widget ) ) {
 
             widget->setBackgroundRole( QPalette::NoRole );
-            widget->setAttribute( Qt::WA_TranslucentBackground );
+//            widget->setAttribute( Qt::WA_TranslucentBackground );
             widget->setContentsMargins( 3,3,3,3 );
             addEventFilter( widget );
 

[10110111]

Also, when the dock widget is floating (i.e. not docked), the paint errors don't occur.