Don't reflow a line as wrapped if it broke on the last cell

[zadjii-msft]

Summary of the Pull Request

If we write a character to the last cell of a row, we'll mark that row as wrapped.

If we linefeed at that point, we'll unmark that row as wrapped. (Linefeeds are important here, we learned in #5291/#5294 that all cursor movements don't actually break a line).

However, in TextBuffer::Reflow, if we increase the width of that row, we'll actually reflow it and the following row as though the first row wrapped. This is due to a iRight < cOldColsTotal that should have been iRight <= cOldColsTotal.

References

• See wsl vim is horribly broken after #5181 #5291/Emit lines wrapped due to spaces at the end correctly #5294 for where we learned about cursor movements and line wrapping.

PR Checklist

• Closes WSL Terminal Line Endings (the "exact wrap" bug) #3088
• I work here
• Tests added/passed
• [n/a] Requires documentation to be updated

Validation Steps Performed

Tested with the following command in WSL

printf "%*s\\n" "$(tput cols)" "" | tr " " "X"

• In conhost, this works like a charm
• This DOESN'T work in the Terminal. Presumably, the last cell getting written like this causes us to print the 'X' line as wrapped, but we never re-render that character as not wrapped when the line breaks. This seems timing related. I've got a test ready, but I want to make sure the test gets all possible cases here, including WriteCharsLegacy ones

I'm drafting this to get feedback on the < to <= fix so far. I'll work on the conpty fix in the meantime

[jsoref]

Suggested change

	// cursor movement actaully broke the line. This test would be much too
	// cursor movement actually broke the line. This test would be much too

[zadjii-msft]

This PR is super old and I'm not coming back to it any time soon. The buffer has changed quite a bit. As noted in #14821 (comment), there might be new ways of fixing this. The buffer's also going through pretty majir rewrites, so best to not have too many cooks.