Quassel allows the hiding of characters off-screen in certain situations.
Paraphrased from Tucos in the IRC channel:
1. "a [bunchofspaces] a" shows 'a' on the first line and 'a' as first char on the second line with all spaces gone;
2. "[bunchofspaces] ab" shows 'b' as first char on the next line.
3. "[bunchofspaces] a" can hide 'a' even with multiple lines of untrimmed whitespace (lucky amount, i guess)
This bug is referencing situation 3.