Steps to Repro:
=> scroll to the bottom left of the editor and notice the horizontal scrollbar is partially obstructing the last line of text
I am not able to reproduce this. Here's concretely how I'm following your instructions:
The cursor now moves to the end of the pasted content, near the bottom of the document, and the last line is visible, above the horizontal scrollbar. If I add a few extra lines so that pasting doesn't put the cursor on the last line, the problem still does not show itself.
This is Firefox 44 Linux. Which OS are you testing on?
Thanks for getting back to me!
I see this on Firefox 43.0.1, Windows 7 and Firefox 41.0.2, Mac OSX Yosemite.
I think you went wrong on step 4 - you should spam letter keys until the Vertical scrollbar appears.
Use a border-bottom on heightforcer to ensure extra hscrollbar space
Ah, sorry for the poor reading. Looks like a bug (or at least strangeness) in Firefox's interpretation of a padding I'm setting. Attached patch seems to work around it. Does it help?