Caret disappears when dash character inserted on very long text in text box

[davidlloyduk]

Using the test.html sample provided in the docs (https://rawgit.com/component/textarea-caret-position/master/test/index.html
), in the manual testing ground on top of page enter the following:

MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM-M

Notice the red display for the caret disappears when you add 'M' after a '-' characted. Remove the 'M' at the end and displays OK again. Only seems to happen when you cause it to scroll with quite a few letters in the text box. Doesnt appear to happen if you have a few characters.

Only found this as it looks to be causing this issue on Github Desktop: desktop/desktop#6806. I've debugged it there and it seems to return an invalid left value for the caret once the dash is added after calling getCaretCoordinates

Found using Chrome 105.0.5195.127

EDIT: Debugged the code from testarea-caret-position and it seems span.offsetLeft/offsetTop is invalid once a dash is added with a new character