The change in appendText(const QString &text):
The original appended to an existing range while you add a new range.
I would think you could just leave the old code untouched, no?

Basically we want to be unchanged, but when it merged, now, it does not have guaranteed that appending to last will be nor same behavior nor correct.

Hmm, I assumed it only got in here when appending a text to
a) an empty shape (no need for merging),
b) at the end of an existing text, in which case using the the existing formatting of the last range should ok. I just can't see why using a defaultfont in this case is correct.

What have I missed?

You are right, when i see docs

/// Appends plain text to the last text range
void appendText(const QString &plainText);