Improve performance when files get really large

[davep]

Populating the disassembly and AST widgets is horribly slow as a Python file gets really large (Textual's widget.py is a great example¹). It would be good to see if there's a way of speeding this up somehow. The issue seems to be coming from how the two base widgets (OptionList and Tree) build themselves up, so I don't know if there's much that can be done, short of building very custom widgets for this. Failing that some sort of incremental update that keeps the display responsive would be a good idea.

Or, worst case scenario: make the display obviously busy.

Footnotes

1. Personally I would never let a file that large near a repo, but it's a good example of a worst case. ↩