Ever since "upgrading" to IE9, I've been having a super-annoying and very inconsistent problem. While editing wikis, every now and then, I can't move the cursor to the previous row. Pressing the up arrow key simply doesn't do anything. Even weirder, moving the cursor to the beginning of the line and pressing the left arrow key causes the cursor to, instead of moving to the end of the previous line, move to the end of the current line. Moving down any number of lines and then moving back up doesn't remove the invisible wall; sometimes in fact it moves the wall to a lower line.
The only way to get around the problem is to click above the invisible wall, which correctly repositions the cursor. The problem then totally vanishes; the cursor is free to move around the entire text box with no problems, until it randomly gets caught up again somewhere else at an undefined later time. The only pattern I can make out is that it seems to happen more often when the cursor is near wikisyntax (
==, and so on). I cannot reproduce the problem intentionally, and I have yet to see it occur on any non-wikis - I have no way of knowing if this is a MediaWiki bug or simply due to different cursor habits causing the problem to never surface elsewhere.