Marks on scrollbar: Highlight mark on scrollToMark

> Unfortunately, the navigation isn't very smart yet. When jumping to the previous/next mark, the window area merely scrolls such that the marked line becomes visible.
>
> It would be _much_ nicer if
> * the line was highlighted (briefly) after the navigation action – this would make it easy to immediately find the marked line visually.
> A temporary highlight effect should be used to visually guide the user to the target line.

How do we reconcile this with FTCS style marks, with a prompt/command/output section? Do we select just the prompt? The command? The whole blob?

What about for multiline marks?
* crawl: a rect with just from the start->end points of the mark, regardless of how ridiculous that looks
* walk: a bunch of rects for each line
* run: rects for each line, with intelligent borders so the border is only around the actual marked text

_originally posted by @jkuebart in #13449_

Start by tracing the scrollToMark navigation entry point and the existing mark-rendering behavior. The issue proposes a temporary highlight for the target line, but leaves FTCS-style and multiline mark geometry unresolved; done would require an agreed visual treatment for those cases.

Written by the indexing model from the issue text.