New
#31
Set up as described, immediately after the capture I get a Save As dialogue.
As for it failing part way thru scrolling, your experience may differ. For example, I'm using build 1709, tried with Waterfox (Firefox derivative). All sorts of possible differences that might have some impact, I suppose.
Scrolling capture seems to be one thing that can easily be made to fail - one person finds one tool ok, another, not.