Paragraph: Font size selector does not work on mobile

[arunsathiya]

Describe the bug

The Font size dropdown selector available on Paragraph block does not respond on mobile devices.

To Reproduce
Steps to reproduce the behavior:

• Start with a mobile browser
• Add a Paragraph block to your post/page
• Click on the block settings
• Click on Font size dropdown

What is expected

The dropdown appears as expected.

What happens instead

The dropdown is not seen.

Screenshots

This was originally reported on Calypso repo - Automattic/wp-calypso#29803

The screenshots have been reused from that report as well.

Smartphone (please complete the following information):

• Device: OnePlus 5, but reportedly reproducible on iOS as well
• OS: 8.1.0
• Browser: Google Chrome
• Version: v71.0.3578.99

Additional notes
I also tried reducing the viewport on Google Chrome Version 71.0.3578.98 (Official Build) (64-bit) for macOS 10.14.2 - Mac OS Mojave, and I see the same behaviour. It works when I launch the browser console though using Option + Command + J - unsure why it works only for this state.

I also haven't checked if this selector is available on any other blocks/affecting any other blocks.

I don't see any previous reports either, but let me know if there is one.