QMainWindow tries to avoid memory allocations by caching tab bars that are no longer used. That cache is populated when the layout is activated. The layout might never be activated when the main window is never shown, in which case adding and removing dock widgets at runtime will continuously allocate more memory for tab bars that are created. A workaround is to call QMainWindow::layout()->activate() explicitly in application code, once all dock widgets have been removed. This change avoids that applications have to do that. Change-Id: I70eb585d2120f0b7b73f59c3ee65d7242c12ec59 Fixes: QTBUG-83135 Pick-to: 5.15 Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>