I could recall, just vaguely, that in some circumstances I couldn’t edit a wiki page, several months ago. But for one reason or another the problem went away and I didn’t notice it again. Then someone reported it, that if you had a module in the pagebottom module zone, somehow the page’s edit textarea couldn’t be accessed; it was covered by something apparently.
This sparked something in my memory, and I checked and realized that the grid column class given to the edit form causes it to float to the left (as with all grid divs). This float allowed the next thing down the page to rise up and conflict with the div, in this case covering it and blocking it from being accessed (SVN r53439).