A Windows popus up when switching to the outline of a QML document

A Windows popus up when switching to the outline of a QML document
(Look at the upper right side of the animation below)

The reason for that is that the toolbuttons in the toolbar are constructed without initial parent. The Filter toolbutton is set to visible before it is parented, and therefore it creates a top-level window. Milliseconds later it gets a parent and the window disappears.