When these panels had arrows, I guess the bottomcenter topleft alignment
made sense so that you could precisely align the arrow, but that's not
what we do now.
Don't use bottomcenter / leftcenter / rightcenter, since we really want
the sides to align.
This shouldn't change behavior on any platform except Linux + Wayland,
where the alignment looks good now in the case of bug 1784876.
Differential Revision: https://phabricator.services.mozilla.com/D156099
72c00adfaa