The source-string assertion windows were too short: the Ctrl+K branch window
(900) cut off before newSession()/closeMobileSidebar() (@992/@1155) and the
Ctrl+B forward window (+120) cut off isContentEditable (@119+). Widen to
1500 / +300 so the guards are actually verified (non-vacuous). Codex-recommended
headroom. Production guard unchanged.