When that happens, it’s because there’s a line of text somewhere that the wiki won’t wrap. There are two possible reasons for that.
To find the culprit, scroll off to far right end of the window, and then scroll down and look for lines of text that extend all the way to the right.
If you find one that’s a link to another website, like this:
take a closer look at that line; the wiki won’t wrap those off-site links, so if this one extends from the left to right margins, it’s the cause of the wider window.
[[http://example.com/garbageblah... | link text to display]]
with the very long address before the pipe (“|”), and something else after it.
The other thing to look for as you scroll down the right margin is text that is formatted as ‘monospace font’ (courier new, usually; it’s uglier). Something like this:
garbage garbage blah blah blah garbage garbage blah blah blah garbage garbage blah blah blah garbage garbage garbage blah blah blah garbage garbage blah
Edit the page and locate that line; you’ll find that it has a space at the *left* margin, before the first character. Remove that leading space and save … and there ya go!