[Carbon] Diacritic input broken, duplicate the base glyph [REG]

Press tonos ('), then alpha (α), then another key (χ), for instance (use any qtextcontrol).
You get: άαχ
Although you just typed: άχ
Seems the second alpha is delivered by an invalid keyPressEvent(),
at least that is the case for my own input widgets.

Update: this is a regression, broken by Commit 8da880e77db04cc4509e0f48e0b5b1d6265da223