PDF Fill - TAB-navigation, not highlighting checkboxes

[cfogelSV]

Affected versions

• 81.0a1 (2020-08-16)

Affected platforms

• Windows 10, macOS 11.0

Steps to reproduce
pdfjs.renderInteractiveForms - enabled

1. Launch Firefox;
2. Access a PDF form with checkboxes;
3. Press the TAB key until reaching and selecting checkboxes;

Expected result

• checkboxes are marked as being in focus;

Actual result

• no indication of checkboxes being in focus

Additional notes:

• S3 as suggested severity.

[brendandahl]

This should be fixed https://bugzilla.mozilla.org/show_bug.cgi?id=1718528