The recent watchlist improvement T50615 should wrap checkbox and adjacent description together into ##<span style="white-space:nowrap">## to avoid checkbox at line end and description starting next line.
The floating arrangement of
<input id="hideminor" type="checkbox" />
<label for="hideminor"></label>
creates a logical (semantic) junction between both elements by ##for=## but does not prescribe where and how the browser shall arrange visible elements within a page; they may occur anywhere.
Best would be if a line break is permitted after first word of description, but that is inpredictable, since label content comes from translatewiki and would cause illegal nesting. ## ## might work on some browsers but is less desirable. Since description texts are supposed to be very short that should fit into one wrapping element for both checkbox and label.
This issue goes for all forms (with floating lines of checkboxes or other form elements) and should be mentioned in GUI guide on mw:.