Ref T13302. The "Close/Cancel" button is currently running two copies of the "dismiss dialog" code, since it's techncally a link with a valid HREF attribute.

An alternate formulation of this is perhaps if (JX.Stratcom.pass()) { return; } ("let other handlers react to this event; if something kills it, stop processing"), but pass() is inherently someone spooky/fragile so try to get away without it.

Opened the Javascript console, clicked "Edit Task" on a workboard, clicked "Close" on the dialog. Before: event was double-handled leading to a JS error in the console. After: dialog closes uneventfully.