Cursor invalidation failing when line renditions are used

[j4james]

Windows Terminal version

1.21.1272.0

Windows build number

10.0.19045.4291

Other Software

No response

Steps to reproduce

1. Open a WSL bash shell
2. Execute the following statement:

   printf "\ec\e[999B\n\e[H\e#6\n\e[1 q"; read
   

3. Type some text and note whether the cursor is blinking.
4. Backspace to the start of the line and check the cursor again.

Expected Behavior

The final escape sequence in that printf is a DECSCUSR command, setting the cursor style to a blinking block, so it should be blinking the whole time (except maybe when it's actually moving).

Actual Behavior

The cursor blinks at the start of the line, but doesn't blink in any other column. The issue isn't specifically related to blinking, though. It can also manifest as almost constant cursor "droppings", but this test case was the easiest to reproduce.

And note that it also fails on openconsole/conhost if you've configured it to use the Atlas engine, so it's not a conpty issue, but it appears to be Atlas only.

Line rendition has got something to do with it - the first line is set to double-width in this test - but it's not only the double-width line that is affected. And I think the scroll offset is also a factor, which is why this test case forces a linefeed at the bottom of the page.

As far as I can tell from a git bisect, it first broke in commit 20b0bed (PR #15500).

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• Conpty cursor mispositioned when using double width lines (#15014), similarity score: 0.77

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• More cursor droppings (#8213), similarity score: 0.76

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[j4james]

It looks to me like the problem is here:

terminal/src/renderer/base/renderer.cpp

Lines 1179 to 1182 in 49e4eea

	const auto coord = _currentCursorOptions.coordCursor;
	const auto lineRendition = buffer.GetLineRendition(coord.y);
	const auto cursorWidth = _pData->IsCursorDoubleWidth() ? 2 : 1;

The _currentCursorOptions.coordCursor value is relative to the top of the screen, while the GetLineRendition call is expecting a buffer row offset.

That said, I don't think we need that call anyway, since the line renditions should already be cached in the _currentCursorOptions structure. Same goes for the IsCursorDoubleWidth call on the next line.

So unless I'm missing something, I'd suggest replacing those two lines with:

const auto lineRendition = _currentCursorOptions.lineRendition;
const auto cursorWidth = _currentCursorOptions.fIsDoubleWidth ? 2 : 1;