CLS should reset the virtual viewport bottom #9872
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Virtual Terminal sequence support
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ghost
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Thanks for this. Sorry for the delay here -- we're getting to the end of a Windows dev cycle and by now you've probably guessed that that tends to eclipse the sun. 😄 I'll revert 9770 on main. |
ghost
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May 2, 2022
The "virtual bottom" marks the last line of the mutable viewport area, which is the part of the buffer that VT sequences can write to. This region should typically only move downwards as new lines are added to the buffer, but there were a number of cases where it was incorrectly being moved up, or moved down further than necessary. This PR attempts to fix that. There was an earlier, unsuccessful attempt to fix this in PR #9770 which was later reverted (issue #9872 was the reason it had to be reverted). PRs #2666, #2705, and #5317 were fixes for related virtual viewport problems, some of which have either been extended or superseded by this PR. `SetConsoleCursorPositionImpl` is one of the cases that actually does need to move the virtual viewport upwards sometimes, in particular when the cmd shell resets the buffer with a `CLS` command. But when this operation "snaps" the viewport to the location of the cursor, it needs to use the virtual viewport as the frame of reference. This was partially addressed by PR #2705, but that only applied in terminal-scrolling mode, so I've now applied that fix regardless of the mode. `SetViewportOrigin` takes a flag which determines whether it will also move the virtual bottom to match the visible viewport. In some case this is appropriate (`SetConsoleCursorPositionImpl` being one example), but in other cases (e.g. when panning the viewport downwards in the `AdjustCursorPosition` function), it should only be allowed to move downwards. We can't just not set the update flag in those cases, because that also determines whether or not the viewport would be clamped, and we don't want change that. So what I've done is limit `SetViewportOrigin` to only move the virtual bottom downwards, and added an explicit `UpdateBottom` call in those places that may also require upward movement. `ResizeWindow` in the `ConhostInternalGetSet` class has a similar problem to `SetConsoleCursorPositionImpl`, in that it's updating the viewport to account for the new size, but if that visible viewport is scrolled back or forward, it would end up placing the virtual viewport in the wrong place. So again the solution here was to use the virtual viewport as the frame of reference for the position. However, if the viewport is being shrunk, this can still result in the cursor falling below the bottom, so we need an additional check to adjust for that. This can't be applied in pty mode, though, because that would break the conpty resizing operation. `_InternalSetViewportSize` comes into play when resizing the window manually, and again the viewport after the resize can end up truncating the virtual bottom if not handled correctly. This was partially addressed in the original code by clamping the new viewport above the virtual bottom under certain conditions, and only in terminal scrolling mode. I've replaced that with a new algorithm which links the virtual bottom to the visible viewport bottom if the two intersect, but otherwise leaves it unchanged. This applies regardless of the scrolling mode. `ResizeWithReflow` is another sizing operation that can affect the virtual bottom. This occurs when a change of the window width requires the buffer to be reflowed, and we need to reposition the viewport in the newly generated buffer. Previously we were just setting the virtual bottom to align with the new visible viewport, but that could easily result in the buffer truncation if the visible viewport was scrolled back at the time. We now set the virtual bottom to the last non-space row, or the cursor row (whichever is larger). There'll be edge cases where this is probably not ideal, but it should still work reasonably well. `MakeCursorVisible` was another case where the virtual bottom was being updated (when requested with a flag) via a `SetViewportOrigin` call. When I checked all the places it was used, though, none of them actually required that behavior, and doing so could result in the virtual bottom being incorrectly positioned, even after `SetViewportOrigin` was limited to moving the virtual bottom downwards. So I've now made it so that `MakeCursorVisible` never updates the virtual bottom. `SelectAll` in the `Selection` class was a similar case. It was calling `SetViewportOrigin` with the `updateBottom` flag set when that really wasn't necessary and could result in the virtual bottom being incorrectly set. I've changed the flag to false now. ## Validation Steps Performed I've manually confirmed that the test cases in issue #9754 are working now, except for the one involving margins, which is bigger problem with `AdjustCursorPosition` which will need to be addressed separately. I've also double checked the test cases from several other virtual bottom issues (#1206, #1222, #5302, and #9872), and confirmed that they're still working correctly with these changes. And I've added a few screen buffer tests in which I've tried to cover as many of the problematic code paths as possible. Closes #9754
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Windows Terminal version (or Windows build number)
74909c0
Other Software
No response
Steps to reproduce
cls.Expected Behavior
The buffer should be cleared and the viewport should be moved back to the top of the buffer.
Actual Behavior
The buffer is cleared, but the viewport remains where it was before the
cls.I'm afraid I broke this with PR #9770. Previously
clswould have reset the virtual bottom when it reset the viewport, but it doesn't do that anymore. And I suspect there are probably more cases like this.I'm really not sure what the right solution is though - maybe every viewport change made through the console API should reset the virtual bottom? But whatever we do, there are going to be some cases that behave weirdly - I think that's inevitable when you're mixing the legacy APIs with VT.
For now, though, I think it might be a good idea to revert #9770, because this bug seems a lot worse than #9754.
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