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Subwindow not shown until I change tabs or show it twice
Hard to know what is happening with just this.
How are you creating that window ?
It looks like you are doing it from another thread, if yes check this sticky.
If not, please copy on the forum the click handler code to see what is happening.
Alexey Java: the problem is that the subwindow is only shown if I click the button twice or if I click the button once and then switch to another tab.
is there some known typical mistake developers make in this area?
Another possibility is that the component triggering the event is not in immediate mode. However, a Button should always be immediate by default so I suspect this is an issue with background threads as Mathias suggested.