Created attachment 396894 [details]
Test case reduction

I've attached a one-file test case reduction. Interactive elements (a button in this case) are responsive to begin with. Click into the <select> dropdown and out again, and now all future button presses take a few hundred milliseconds to respond. This persists even after a page refresh; only a force refresh seems to fix it.

The delay happens to all interactive touch/click events (ie, links, buttons etc). In the test case, the button (toggling the background color) just makes it easier to appreciate the delay, but the delay also applies to opening the <select> dropdown itself.

I've put the same test case here: https://jamielinux.github.io/webkit-select-font-size-bug/

Steps to reproduce:
1. Create a <select> dropdown.
2. Style <option> font-size to any vmax value.
3. Load website. 
4. Touch events (eg, a button) are responsive.
5. Open the <select> dropdown and close it.
6. All future touch events have a delay of a few hundred milliseconds.
7. Refresh page and problem still persists.
8. Force refresh page and problem resets.

Hardware/software details:
- iPhone XS Max
- iOS 13.4.1
- Both iOS Safari & iOS Firefox