As requested in Bug 1727072 i file this bug.

Context menu frequently appears like this.

The above comment and this one happen when you click towards the bottom of the viewport with little space below it for the context menu to appear.

So the above happens but sometimes it appears on top of/above the cursor - like it should.

This last case is what happens when i click somewhere close to the top of the viewport - it shows the scrollbar even then, so maybe what the issue is/what happens its an erratic behavior between two ways of working - one which tries to position the context menu inside the viewport and therefore generates scroll arrow; and another one, which doesn't care about the viewport and if it happens it also draws part of the context menu over the address bar so that it doesn't produce a scroll arrow. This is all speculation from myself.

The popup position is out of our control so closing as wontfix.

Filed https://bugs.kde.org/show_bug.cgi?id=442304

This comment applies to this bug.
@Martin please reopen this bug as i don't have privileges to do so. It's not an urgent bug at all so place it wherever you find appropriate in your backlog.

It's already reopened.

Can you please attach whole screen with the broken popup there?
Thanks.

Comment from Vlad:

Firefox sets the following xdg-positioner constraint adjustments: slide_x, flip_x, flip_y, resize_x, and resize_y. If a popup can't be flipped vertically, the only thing that's left is to resize the popup (SLIDE_Y is not set). It's not a compositor bug, firefox needs to set appropriate constraint adjustments.

Attaching.

Can you please test latest nightly?
Thanks.

Still happens in Firefox Nightly 100.

Does this problem still occur with Nightly?

Yes