Start writing a message in the quick reply,
add a code block with a line with tabs at the beginning,
* click preview,
all the tabs are stripped out.
ETA: apparently are converted to one space irrespective of the initial number of tabs.
ETA2: okay, not always reproducible, I think it depends on the browser...guess which one is broken? O:-)
ETA3: ...more funny, I went back and the tabs were gone... I feel it's the editor.