toolbars below content: use linux top margin hack to extension panels We already apply computed top margin to notification popup and permission popup so that they are visible. But it seems that if the panel is close enough to screen bottom edge (such as in maximized windows) then the panel contents fail typically fail to render. For whatever reason, changing the default margin for customizationui-widget-panel makes the failures much less frequent although it doesn't appear to get rid of the issue completely.