Improve code context menu layout position esp visual stability (#22102)

* Now decides whether the menu is above or below the target position
before rendering it. This causes its position to no longer vary
depending on the length of completions

* When the text area is height constrained (< 12) lines, now chooses the
side which has the most space. Before it would always display above if
height constrained below.

* Misc code cleanups

Release Notes:

- Improved completions menu layout to be more stable and use available
space better.