Prevent opening of panel if panelbeforeopen callback calls preventDefault()

[jtara]

It would be useful to be able to prevent panels from opening by calling preventDefault() from a panelbeforeopen callback.

[gabrielschulhof]

That's not a bad idea, however, there's a bit of a problem: panelbeforeopen is used by an open panel to decide to close in order to make way for a panel that is about to open. If we allow the default to be prevented, the old panel will receive the signal before the third-party code has had a chance to prevent default - unless the third-party code attaches a handler below the document level, which is where the panel's handler is attached.

[gabrielschulhof]

I guess it's not a "problem" per se, but something to keep in mind when writing the preventable panelbeforeopen event.