Partial regression of #24393, not only regression, but broken for long
time, 24393 didn't really improve it but used wrong `overflow: scroll`.
Actually, that "ui secondary filter menu labels" shouldn't be set as
scrollable (I missed that at that time), the problem is: if a "ui menu"
has "dropdown" items, then it should not be scrollable. Otherwise the
dropdown menu can't be shown correctly.
And there are more problems:
* The "issue-filters" shouldn't be used anywhere else (copying&pasting
problem again ....)
* There is also an "issue-actions" container, it should also be fixed.
* There are similar problems on the milestone page.
* The old comment in code: "grid column" doesn't work well.
The major changes of this PR are: use "flex: 1" instead of "ui grid
column".
After this PR, not 100% perfect but much better than before.
silverwind
