I am still looking, these are my findings so far. I think how Discord's input box works is, the input box is a contenteditable div and they listen the `beforeinput` event to catch which key they user has pressed. They also use `preventDefault()` to not letting the browser updates the input box, instead they'll use `editor.textContent` to update the value manually. And then they'll use some other javascript to manually update the cursor position. Perhaps some of the changes caused some confusions to their cursor position updating code. This is what I am still investigating.
Bug 1839660 Comment 8 Edit History
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I am still looking, these are my findings so far. I think how Discord's input box works is, the input box is a contenteditable div and they listen the `beforeinput` event to catch which key the user has pressed. They also use `preventDefault()` to not letting the browser updates the input box, instead they'll use `editor.textContent` to update the value manually. And then they'll use some other javascript to manually update the cursor position. Perhaps some of the changes caused some confusions to their cursor position updating code. This is what I am still investigating.