Scrollable outputs are not keyboard-accessible

[choldgraf]

thebe-core makes cell outputs scrollable but not focusable (e.g. tabIndex=0), so keyboard users can't scroll wide outputs (e.g. pandas DataFrames, wide plotly plots).

I think the problem is that this sets an overflow, but we don't set tabIndex etc on the elements that it applies to:

thebe/packages/core/src/index.css

Lines 74 to 77 in c69aae7

	/* Overides for jupyter styles */
	.thebe-output .jp-OutputArea-child > .lm-Widget {
	overflow: auto;
	}

How to fix

I think we'd need to dynamically sense whether the output is going to overflow, and apply tabIndex=0 and role="region" etc if it does, either with a MutationObserver or ResizeObserver.

Notes

• We first ran into this with @pancakereport's axe checker via Make cell outputs focusable if they aren't stdin/stderr myst-theme#864