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While exporting a custom panel I clicked in the name field and suddenly the whole interface turned grey, except for the text-field. I found the invisible OK button, through clicking around in the grey. After that everything was visible again.
Today I double-clicked a layer to rename it. The name-input-field popped out, but everything else turned grey again. This time I found the top-bar of the window and moved it a bit, what looked like this I know this kind of glitches only from windows, but never saw them on a Mac.
By the way I'm using TVP 11 Pro and OS X 10.10.2

did you use a specific german letter ?

No I never use umlauts or stuff like this when naming files, layers, folders etc. cause it always leads to problems.

It's a well-known bug that's hard to fix. It happens when you hit some specific keys on your keyboard, such as ^ or ¨ on French keyboards.

To finds your interface back, you just have to resize TVPaint's window.

Ah good to know