It kinda looks like it's being hidden by the search bar...
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Funnily enough, reducing the window width even further makes the top button reappear:
View attachment 25580
(Firefox 61.0.2 on Windows 10 x64)
The CSS has a number of "common sizes" defined and when those dimensions are satisfied the CSS adjustments come into effect. I personally am not a huge fan of segmenting in this way, and would prefer that various things change size, and only when that is ineffective, should things rearrange or be omitted. I also do not design CSS the way many suggest I should, that is to favor small form-factors and then build upward. I do the reverse, I favor adapting to the largest screen (such as a 1472x1017 browser window) and test to see how it appears at smaller dimensions. Where necessary, I make changes, and if at all possible I would keep them limited in number so that the bulk of the page would render unchanged.
I cannot say because it seems that most often complaints relate to the 'default' theme here, but I would hope that my themes do not have such hiccups.