On en.wp (currently running 1.28.0-wmf.6 (d50d367)), the "Search for contributions" box where one selects username, namespaces, various checkboxes, etc has no breakable whitespace in each line of options. As a result, the widest line sets a mandatory width of that boxed part of the display. If my window is narrow, it overflows the window width. I'm using Firefox-47.0 on OS X, with Wikipedia's Vector skin.
There is a between each checkbox and its label to keep them together (good!) and each checkbox+label is contained in a mw-input-with-label span that has white-space:nowrap in its CSS, which further reinforces no breaking anywhere in that chunk. But there seems to be no breakable whitespace between successive checkbox+label spans in a row.
Compare to Special:Whatchlist, where there is optional line-break possible between each checkbox+label in the "Hide:" row of options, which avoids overflowing my window width. If my window is narrow, that list of options simply wraps onto a second line.