Thank you for the explanation Alex. I was not necessarily looking to circumvent any such restriction, just have control over the text and placement of the error message - in general we consider a popup alert refering to "Lightstreamer" (which the end user may or may not have any knowledge of) is less than friendly. Feeding the error through the normal reporting callbacks already in existence seems preferable - and you can still refuse to start the engine as now.

I will go investigate the alternate engine creation strategies you pointed out, I'd only just my code converted to the loadEngineMinimal method, now I notice it is deprecated!

It is not clear to me what all the consequences of supporting sharing an engine across windows are, especially if the original push page that loaded it is closed. It makes sense for frame based apps, but if multiple browser tabs or windows are open - I'm just not sure.