FIX: Composer height issue in iOS 15 Safari (PR #14282)

Safari on iOS 15 has an address bar pinned to the bottom of the viewport. When the software keyboard is invoked, this bar floats above the content, hiding the buttons in the composer.

This PR adds bottom padding for Safari iOS15 only, the issue is not present on other browsers in iOS (like Chrome) or in DiscourseHub.

At this time, there is no way to detect Safari’s floating address bar. Apple has recognized this is a known issue in this Webkit bug report: 229876 – iOS 15 Beta 8 VisualViewport.height does not account for address bar above virtual keyboard

See also: iOS 15 Safari detect floating address bar when keyboard is visible - Stack Overflow

GitHub

How does this look like on iOS 15 with Single Tab setting enabled?

It has some extra padding between the keyboard and the submit buttons.