Measure loop while scrolling in a scaled editor

[viorgu]

Describe the issue

The editor seems to get stuck in measure loops when scrolling while scaled and the loop is forcefully stopped with the "Measure loop restarted more than 5 times" check.

The issue can be easily reproduced adding a transform: scale(0.5); to the parent element of the editor on a demo page like: https://codemirror.net/examples/million/ and observing the console while scrolling fast or holding the Page Up/Down keys.

From what I can tell the issue is that getBoundingClientRect() returns slightly different values while scrolling (probably because of rounding errors in BlockGapWidget height) and the scale computed in https://github.com/codemirror/view/blob/4e355eab50de94ab315ed293729f5365841fe3c8/src/viewstate.ts#L258 is also different so measuring continues at https://github.com/codemirror/view/blob/4e355eab50de94ab315ed293729f5365841fe3c8/src/editorview.ts#L427

Rounding the computed scale to 2 decimal places seems to prevent issue and should also remove the need for the other range checks in getScale.

An alternative fix would be to round the gap height at https://github.com/codemirror/view/blob/4e355eab50de94ab315ed293729f5365841fe3c8/src/docview.ts#L488 to an integer.

Unfortunately I'm not familiar enough with the project to know if the loss in precision from these changes would cause other issues or if there's a more appropriate fix.

I can open a PR with any of these options if it helps.

Browser and platform

MacOS & Windows / Firefox & Chrome

Reproduction link

No response

[marijnh]

Indeed, that shouldn't be directly comparing floats like that. Attached patch adds a margin of .005 in which it doesn't treat the value as changed.

[viorgu]

Would it make sense to check if the computed scale is 1 as well?
Otherwise it looks like there's a chance the scale might never reach 1 again if there's an animated reset.

[marijnh]

Any scales close to 1 are already rounded to that in getScale, so I don't think this will be an issue.

[viorgu]

I missed that, thanks!

[gquittet]

@marijnh The issue is still there when you use the scrollPastEnd plugin

[marijnh]

@gquittet Can you provide a reproduction script for that? I couldn't get it to happen even with scrollPastEnd.

I did notice that extension didn't work right in a scaled editor, though. See attached patch.