da6f3a668758b8997baa758a6671e7d560d3caf3  [Site Isolation] <input type='checkbox'> is not greyed out when window becomes inactive

Classified as not security-relevant.
Triage note: FocusController::setActiveInternal switches localMainFrame() to localMainOrRootFrame() so a local root frame (remote main frame) still updates control tints; site-isolation correctness, no memory-safety bug.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+4.87), null_guard (+1.2), surgical (+1.3)