Preserve the cursor row during Clear Buffer

[lhecker]

Introduces an ABI change to the ConptyClearPseudoConsole signal. Otherwise, we have to make it so that the API call always retains the row the cursor is on, but I feel like that makes it worse.

Closes #18732 Closes #18878

Validation Steps Performed

• Launch ConsoleMonitor.exe
• Create some text above & below the cursor in PowerShell
• Clear Buffer
• Buffer is cleared except for the cursor row ✅
• ...same in ConPTY ✅

[DHowett]

wow, we actually have tests that this caused to fail!

[DHowett]

Still broke during tests (and now audit! may need to merge main)

[DHowett]

oh you merged main. alas

[DHowett]

PowerShell and CMD both resume writing input at the old cursor position after we clear the buffer this way:

[DHowett]

Clearing the buffer this way after the viewport scrolls down results in a loss of the prompt:

[lhecker]

Right, forgot about the tests.

PowerShell and CMD both resume writing input at the old cursor position after we clear the buffer this way:

That's already the case before this PR, though.

[DHowett]

That's already the case before this PR, though.

Huh. Was it the case before 1.22/the conPTY rewrite?

[lhecker]

Huh. Was it the case before 1.22/the conPTY rewrite?

No, I mean just before this PR, on main. It did work in 1.21 because the CMD prompt relied heavily on the conhost TextBuffer. PowerShell 5 is broken in 1.21 too, though.

[lhecker]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[j4james]

FWIW, it still destroys the content of the line when it is the last line on the screen.

This is the off-by-one error I mentioned in the branch here:

// Erase any viewport contents below (but not including) the cursor row.
if (viewport.Height() - cursor.y > 0)

I think that test should be viewport.Height() - cursor.y > 1.

[lhecker]

Oh that's how to repro the off-by-one issue. Duh. It's really obvious in hindsight. Even though you (j4james) described it correctly, I misunderstood it as "doesn't clear the last line of the viewport" somehow. Too much context switching. 🫠

Edit: Ironically I even rewrote the if condition so I could use > 1 and then promptly forgot about it…