[Bug] Cursor jumps to next line start when tapping end of wrapped line after pinch-zoom on touchscreen

[lishuangfeng]

Describe the issue

When lineWrapping is enabled in CodeMirror 6, after performing a pinch-to-zoom gesture on a touchscreen device, tapping on the blank area at the end of a wrapped line causes the cursor to be positioned at the start of the next line, rather than the end of the current logical line.

Browser and platform

HUAWEI Browser

Reproduction link

https://codemirror.net/try/

[lishuangfeng]

Tapping on the blank area at the end of the fifth visual line of logical line #1, the cursor appears at the start of the sixth visual line of logical line #1.

[marijnh]

Though CodeMirror has a concept of selection associativity, and the browser does as well, at least internally, the JavaScript selection API does not provide access to that. So when the user selects via tap, all we see is a selection update to a specific offset in the document, but we don't know whether the selection was put before or after the wrap point.

I was hoping that Selection.getRangeAt(0).getBoundingClientRect() would allow us to guess the actual position of the selection, but it appears that, at least in Chrome Android, that always points before the wrap point, even when you put the cursor after it.

I've been thinking about trying to move touch selection into the library, rather than letting the browser handle it, also because of some other issues. Unfortunately, this is also problematic, since some native behavior, such as showing the native context menu, cannot be replicated from JavaScript.

Maybe we need some hybrid approach, where we observe touch events, let the native behavior happen, and then interfere only when necessary. In this case, I guess registering where and when the last touch happens, and then using that to guess the associativity of the new cursor when the selection changes briefly after that touch event, might work. I'll look into it.

[marijnh]

Attached patch does something like what I described, and seems to work pretty well.

[lishuangfeng]

Could you please let me know if the examples on the official website (https://codemirror.net/try/) are up to date with the recent code changes? Just to add some context: the issue I described occurs when using pinch-to-zoom and then tapping on a touchscreen. Thanks! @marijnh

[marijnh]

The website is only updated when a package is released. The patch above hasn't been included in a release yet.

[lishuangfeng]

Thanks for merging this! Really appreciate it.
Any rough idea when this fix might be available in a release? We're eager to try it out.
Thanks again!

[marijnh]

I've tagged @codemirror/view 6.39.17