[BUG] Dropdown list is not constrained to screen

[1j01]

If the terminal is small enough that the max number of items doesn't fit on screen, the bottom of the dropdown is cut off, making the the bottom of the scrollbar and the last few items inaccessible.

Screencast:

Also the scrollbar appears to be half-width, probably cut off horizontally. (Maybe it the scrollbar width should be subtracted?)

I guess this project predates the overlay and constrain styles added in Textual 0.24.0 and could benefit from them, although they're still undocumented.