Various combining marks (U+0300 - U+036F) are incorrectly rendered using monospaced fonts.
Attached is a file I used to test which marks where affected (UTF-8 encoding).
I have tried a dozen fonts including the multiple Liberation and DejaVu variants and all monospaced fonts show the same rendering bugs whereas the variable width one do not.

Update : see attached scrennshot of the attached file. On the left, a QTextLayout-based editor with DejaVu Sans Mono, on the right the same editor with DejaVu Sans. Notice how some (but not all) combining marks are improperly rendered (i.e not combined) with the monospaced font.