In my case: I'd left the IDE running for at least a couple of days, and was doing some tidying up yesterday. I used "search all files" several times in succession, and something like the second and third attempts only presented a black window (border etc. roughly as expected) while the first and subsequent were OK.
This wasn't the start of the day, and the screen savers etc. were disabled. I /might/ have seen one or two other programs on occasion behaving similarly although I can't remember enough to reproduce things.
Desktop in this case is KDE, but Lazarus had been built locally using GTK2 ("bigide").
I suspect that this particular one is recent, i.e. only with the upgraded GTK. As I've mentioned in another thread I've seen problems with the previous version where the editable text field of a listbox wasn't being updated (left white until forcibly redrawn), and I suspect that there's a related problem still in some dialog(ue)s where the filename part is left white but the extension is OK.
MarkMLl