Double check how we reflow for a huge height delta

[zadjii-msft]

Yes, exactly that. I did it for this test specifically:

terminal/src/buffer/out/ut_textbuffer/ReflowTests.cpp

Lines 294 to 315 in fc4a37e

	TestBuffer{
	{ 6, 5 },
	{
	{ L"ABCDEF", false },
	{ L"$ ", false },
	{ L"GHIJKL", false },
	{ L"MNOPQR", false },
	{ L"STUVWX", false },
	},
	{ 0, 1 } // cursor on $
	},
	TestBuffer{
	{ 5, 5 }, // reduce width by 1
	{
	{ L"F$ ", false },
	{ L"GHIJK", true }, // [BUG] We should see GHIJK\n L\n MNOPQ\n R\n
	{ L"LMNOP", true }, // The wrapping here is irregular
	{ L"QRSTU", true },
	{ L"VWX ", false },
	},
	{ 1, 1 } // [BUG] cursor moves to 1,1 instead of sticking with the $
	},

$ 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:

terminal/src/buffer/out/ut_textbuffer/ReflowTests.cpp

Lines 294 to 315 in 6374aec

	TestBuffer{
	{ 6, 5 },
	{
	{ L"ABCDEF", false },
	{ L"$ ", false },
	{ L"GHIJKL", false },
	{ L"MNOPQR", false },
	{ L"STUVWX", false },
	},
	{ 0, 1 }, // cursor on $
	},
	TestBuffer{
	{ 5, 5 }, // reduce width by 1
	{
	{ L"$ ", false },
	{ L"GHIJK", true },
	{ L"L ", false },
	{ L"MNOPQ", true },
	{ L"R ", false },
	},
	{ 0, 0 },
	},

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 #15701 (comment)

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?