We don't support any browser needing this for very long: https://caniuse.com/?search=selectionStart
I'm keeping some protection so It doesn’t crash but ultimately `element.selectionStart` should be enough.
Im not removing this in the commit, but the `caret_position.js` file seems barely used.
Also corrects the positioning of autocomplete (when typing @ or emoji)
Previously there were edge conditions where autocomplete would be hundreds
of pixels away due to a bug measuring.
This correct an issue where Firefox ends up having an enormous blank space
at the bottom of topics after editing.
