Characters partially disappear when zooming on iOS devices

Created by: loopspace

Issue Summary

When using "pinch to zoom" on an iOS device (iPad/iPhone) on a page with MathJaX rendered as "CommonHTML" (such as the MathJaX test page) then characters flicker and if the zoom is stopped in the right place then they can (partially) disappear.

This does not happen with the SVG renderer.

Steps to Reproduce:

1. Open up the samples page https://www.mathjax.org/#samples on an iOS device - I tested with safari and chrome. Make sure it is set to render as "CommonHTML"
2. Use the "pinch to zoom" facility to zoom in on a piece of Mathematics. You should see a noticeable flicker.
3. You can "catch" the flicker in the act if you stop the zoom carefully.

Although most of the time it's really obvious that something weird is happening, if the zoom is just right then it can turn a plus into a minus.

Technical details:

• MathJax Version: 2.3 (latest commit: f3aaf3a2)
• Client OS: iPadOS 13.4.1, also confirmed on an iPhone (iOS version unknown)
• Browser: Chrome 81.0.4044.124, Safari (latest version - can't find version number)

Supporting information:

Screenshots from the MathJaX samples page.