Double check how we reflow for a huge height delta

Yes, exactly that. I did it for this test specifically:
https://github.com/microsoft/terminal/blob/fc4a37ee9189f2f7335ab942d50381a38a03acf2/src/buffer/out/ut_textbuffer/ReflowTests.cpp#L294-L315

`$` is the cursor. `false` in each line means that it has an explicit newline (`true` = force wrap).
It has a lot of comments about bugs in the old implementation, which is why the entire text past the cursor fits into the new buffer. In the new, fixed implementation the `false` = explicit newlines are correctly handled and so it doesn't fit anymore:
https://github.com/microsoft/terminal/blob/6374aece11edbf8b1329d56ab963e6ea2089f83d/src/buffer/out/ut_textbuffer/ReflowTests.cpp#L294-L315

Basically I had to make a choice over whether I:
* continue copying until all text in the old buffer has been consumed.
If the new cursor position is now somewhere at a negative Y, we just put it at (0,0).
* stop copying at some point to prevent the cursor from being lost.

I chose the latter. What do you think we should do?

_Originally posted by @lhecker in https://github.com/microsoft/terminal/pull/15701#discussion_r1332947703_


> > continue copying until all text in the old buffer has been consumed.
> > If the new cursor position is now somewhere at a negative Y, we just put it at (0,0).
>
> Probably that one, right? Like, the priority should be to keep the newest text, right?
>
> ... this is currently a 1.19 candidate. I'm okay to say merge as is and iterate in the first hotfix since
>
> 1. we're out of time and I'm about to log out for the day
> 2. this is a RARE edge case
> 3. this is more goodness than badness)

> This is an interesting case. I'm honestly not sure how to handle it -- we should look at how other terminals that do reflow handle it?

Start with the reflow case in src/buffer/out/ut_textbuffer/ReflowTests.cpp at lines 294-315 and compare the two referenced revisions. Determine whether reflow should consume all old-buffer text or stop to preserve the cursor, then update the implementation and test so the chosen behavior is explicit.

Written by the indexing model from the issue text.