Highlights the wrong words with non-standard font size

**Describe the bug**
This is for the Firefox extension (is this the right place for it?). The spellcheck highlight boxes are completely off when using something besides 100% scaling for font size.

**To Reproduce**
Steps to reproduce the behaviour:
1. Go to Firefox settings, then accessibility.
2. Change font size to 140%. (Edit for clarification: You change the default zoom, then tick font only.)
3. Open a new tab and write somewhere, for the best effect, more than one line.
4. See error

**Expected behaviour**
The highlights to be over only the correct (or I guess incorrect) word or section.

**Screenshots**

**Platform**
Firefox extension.

Start by reproducing the Firefox extension issue with Firefox font-only scaling set to 140%, using a new tab with multiple lines of text, then trace the spellcheck highlight rendering entry point. Done means highlight boxes remain over the correct words or sections at non-standard font sizes.

Written by the indexing model from the issue text.