Rebased to HEAD and all suggestions from code review have been applied.

Thanks for the update! I just tried using it actually and it's a bit rough around a few edges:

1. The unblended highlight colour is not a good choice for the background; its intended use is for foreground elements. I would suggest blending it with the base color or use RectangularGlow or another kind of frame to mark the edited/replied message.
2. Pressing some keys (Esc, e.g.) when editing/replying removes the HUD text; the icon, the message selection, and the actual mode remain though. I would suggest using just one way to convey the mode (and I think the QToolButton next to the entry is better for that). And please make Esc actually reset the mode - it's the way Element Web works, in particular.