On-screen keyboard works neither in Tor Browser nor in Thunderbird if started after it

Originally created by @intrigeri on #14752 (Redmine)

This problem probably has the same root cause as #9260 (closed). Gedit is not affected so it must be related to something weird we do for Tor Browser specifically.

So far, this got reported (#10263 (comment 71653)) only by the people who participated in the 3.2 manual testing session, so I’ll assume that very few people use this feature at all. Still, the on-screen keyboard is an important accessibility and hardware enablement (touch devices) feature so I’ll handle this as a regression, not with super high priority but still. If it’s too hard to fix quickly I’ll document this as a known issue for the time being.

Feature Branch: bugfix/14752-a11y-enabled-after-tor-browser-is-started

Related issues

• Related to #9260 (closed)
• Blocked by #14735 (closed)