Completion menu flashes on keystrokes

[AndiLavera]

The completion menu is fast but renders several different states in milliseconds, resulting in flashes on each keystroke.

See video:
https://github.com/user-attachments/assets/045f7508-1a75-4aeb-b2ae-f12cf53bdecf

Screenshots

Right after typing a before acm has updated

First state change:

Notice the width of the completion choices grew.

Second state change

Popup documentation grew and is rendering markdown

Final state

Popup documentation rerenders, no longer markdown

Potential Solutions

For growing/shrinking, set a width for the menu and popup documentation so they stay constant, regardless of content.
For the documentation popup, maybe we can defer the rendering until the markdown rendering has completed?

[manateelazycat]

PR are welcome!