The numbered context-surface switcher (mod+digit) fired even while focus was
in an editable target, stealing the browser's own tab-switching chord and
opening the changes pane mid-typing (issue #2503). Guard the digit branch
with an editable-target check (input/textarea/contenteditable, covering the
CodeMirror composer) so the chord keeps its normal meaning while the user
types; surface switching still works from any non-editable focus, and the
shortcut remains rebindable/unassignable in Settings.
Fixes#2503
Serhii Dziupin
f64c4a74af