[Qt-interest] size of a widget with layout before shown

[Frank Mertens]

Frequently I face the same problem with Qt's layout system.
How to determine the size a widget would take if shown as top-level widget?
Let's say a QMenu? -- Need to have it placed relative to its size.

I'm used to hack around this issue by show()/hide() and some forced
manual event processing.

But is there no clean solution?