Consider ability of the user agent to force unlock

[chrishtr]

Examples that might justify force-unlock:

a. In mode 1 (see #12), and the subtree has been locked for "too long"
b. The user wants to perform actions such as find-in-page and the subtree has been locked for "too long", where here "too long" is less than (a).
c. Detected starvation of commit due to script continuously mutating DOM during commit which prevents completion of the cooperative commit (this one would force a sync commit, not "force-unlock").

[chrishtr]

More reasons to force-unlock:

• Entering fullscreen or modal dialog (reported via https://discourse.wicg.io/t/proposal-display-locking/2905/6)

[vmpstr]

obsoleted by rendersubtree values.. specifically activatability. No timeout in the declarative version