Keep the label as it was: "Take a New Screenshot"

You forget to set the window title back. It stays at e.g. "2 seconds" if I cancel

I would force it to Qt::ToolButtonTextBesideIcon however I tried with diferent styles and it always looked like a QPushButton, so maybe that behavior is only relevant for when inside a QToolbar

Unrelated change

Clever trick I must say :)

Coding style. But this can probably just be forward declared here class QAction;

While the coding style is ... interesting, I would keep it and the new variables consistent with the rest for now

I thought given enough time maybe every variable will be touched and the style changed

It looked so bad I thought I had accidentally changed it

Can't you just call restoreWindowTitle() in any case?

What is this supposed to do? Do you mean pixmap.isNull() or pixmap.size().isEmpty()? Looks unrelated to the patch, though.

Can you perhaps move the setWindowModified() handling into this method, too?

Thinking about it, would using an enum make this nicer?
setButtonState(State::Cancel) or (State::TakeNewScreenshot)` instead of having this one dedicated method for one case but the other one scattered around the code

Unused

No because if you saved a screenshot and then take a new one we want to the title to be "Unsaved" and not the filename of the previous saved file.

In this patch I also changed that we go through this function if the region selection is canceled (to reset title and and button).
I tried to move the resize call into the above if but it produced different results and the size was off. This is just another condition that is false when the above happened. I agree it would be better to change it to the same condition as above.

Do you think of making this a general method to handle windowModified and title setting? Because otherwise I see no need because windowModified doesn't change when we restore the title.