Mobile overflow

[lewisgoddard]

Fixes #4011

Changes Summary

• Fixed horizontal overflow on narrow mobile viewports caused by the hidden .popover-content extending beyond the page width
• Popover is now clamped within the viewport when opened, with the arrow repositioned to stay aligned with the trigger
• Fixed the Language button toggle: clicking it a thirdtime now opens the popover correctly
• Removed unnecessary body overflow locking that was hiding the page scrollbar

This pull request [ is ] ready for review.

[RMcNeely]

LGTM