The sidebar registers a global window keydown listener that expands/collapses the selected mailbox's subfolders on ArrowLeft/ArrowRight. It didn't check where focus was, so pressing Left/Right while typing in a new email (the contentEditable composer, the subject field, search, etc.) toggled the inbox's subfolders open/closed.
Bail out of the handler when the event target is an editable element (INPUT/TEXTAREA/SELECT/contentEditable). Folder-tree arrow navigation still works when focus isn't in an input.
