I believe what's happening here is that there's a partially-visible list menu item, and when you hover the extreme visible top/bottom edge, you end up hovering that partially-visible menu item, and it snaps into view so that you can see what you're hovering.
I can reproduce this in XUL dropdown widgets (i.e. in `about:preferences` as described here), but not in web-exposed HTML content dropdown widgets -- i.e. not with this testcase:
> data:text/html,<select><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option>
So this is a XUL-formcontrol-specific issue, as far as I can tell, and probably not something that's likely to be fixed, given that we're trying to migrate from XUL to HTML&CSS for all of our frontend code.
Bug 1609921 Comment 4 Edit History
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I believe what's happening here is that there's a partially-visible list menu item, and when you hover the extreme visible top/bottom edge, you end up hovering that partially-visible menu item, and it snaps into view so that you can see what you're hovering.
I can reproduce this in XUL dropdown widgets (i.e. in `about:preferences` as described here), but not in web-exposed HTML content dropdown widgets -- i.e. not with this testcase:
> data:text/html,<select><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option><option>a</option><option>b</option><option>c</option><option>d</option><option>e</option><option>f</option><option>g</option><option>h</option>
So this is a XUL-formcontrol-specific issue, as far as I can tell, and probably not something that's likely to be fixed, given that it's always behaved this way & we're actively trying to migrate off of XUL & onto HTML&CSS for all of our frontend code.