The on-screen keyboard does not work in Thunderbird on feature/buster (regression)

The logs say: (thunderbird:7855): CARIBOU-CRITICAL **: 12:05:16.696: file caribou-gtk-module.c: line 1028: unexpected error: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.Caribou.Keyboard was not provided by any .service files (g-dbus-error-quark, 2) (https://salsa.debian.org/gnome-team/caribou/blob/debian/master/modules/gtk3/caribou-gtk-module.c#L1028). The on-screen keyboard works fine in Thunderbird in Tails 3.13.2.

Feature Branch: bugfix/16756-fix-on-screen-keyboard-in-thunderbird-and-remove-caribou

Related issues

Blocks #16628 (closed)
Blocks #16209

Original created by @intrigeri on 16756 (Redmine)

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