[...]
And the answer from Neil is:
This feature was actually contributed by ActiveState.
We have had a report of an infinite loop in Geany when an attempt is made to draw a block Caret on a zero width space character (u200b).
The backtrace makes the problem appear to be in DrawBlockCaret so you could switch to the standard line caret.
There are features in Scintilla that I have accepted despite thinking they are a poor idea, such as the block caret. Perhaps the people that like this feature can work on fixing it.
Cheers Lex