Dragging vim.exe across the DPI boundary to a 100% scale display corrupts the first line of the buffer - maybe related to #5398?

Probably not related to #5398, but generally #5181/#5294. I'd bet it repros in master too. The exact repro is:

1. Open vim.exe with no profile
2. Create a file with two lines: "aaa" and "bbb". I didn't have a trailing blank line.
3. Move the cursor to the 'a' in the first column, so the cursor is at 0,0.
4. Decrease the width of the buffer by one char.

At this point, the buffer will get corrupted. The "aaa" line will be in the scrollback, we'll have two "bbb" lines in vim, but the conpty will have the correct text.

Found during #5345/#5427. I also had #5398 merged into my branch at the time.