Fix find_and_replace div positioning; handle resize; fix lil bugs; add specs

What does this MR do and why?

Closes Handle textarea resize when searched text is hi... (#524289 - closed).

We fix a few things:

• Fix the positioning. It was offset by 2px vertically for Reasons™ I detail in the (human-authored!) message in the first commit.
• Handle resize; i.e. the ask of Handle textarea resize when searched text is hi... (#524289 - closed).
• Fix a little bug when matches break in the textarea but weren't in the clone div.
• Add some specs.

Screenshots or screen recordings

What	Before	After
Find, and resize
Find with match breaking over line	*See below.

* N.B.: This "Before" is between commits 2 and 3 in this MR, when I noticed this issue. You can't observe it with a resize unless you do the same (since the resize fix only came in commit 2, and then I fixed this in commit 3), but it reproduces on master if you have the match words wrapping before you start your search.

How to set up and validate locally

1. Enable the find_and_replace FF on your GDK.
2. Open the plain-text Markdown editor with a document that has stuff to find.
3. Cmd-F (Linux/Windows: Ctrl-F) and search; note it looks wrong.
4. Resize the window; watch the matches disconnect from their source!
5. Check this branch out.
6. Repeat 2-4 and note that things look right.
7. Try with a match with a space in it, and resize to have it wrap halfway through; it should Just Work™.
8. Birds 🐦‍⬛

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