Type something that produces omnibox dropdown entries with associated keywords (ideally not in the top slot, since it's more obvious this behavior is wrong when the first keyword is in the second slot).

Tab until you're inside one of the keyword entries.  Hit esc.

Expected: Revert to the original typed text with top line selected, just like what happens if you tab down to a row that doesn't have a keyword.

Actual: Editing is cancelled entirely.

I suspect this is because while in keyword mode the user text is empty, and we treat esc-on-empty-user-text as "bail out of edits" without realizing that we're really more in the "temporary text" case.  This may be tangentially related to bug 736931.