Behavior is the same in version 7.1.3, too.

The browsers are blocking a popup legitimately caused by a user event (in this case, originating from Flash though). It's a bug in the browsers really. Obviously it would be preferred to work around, but we can't rewrite the UI in a patch release, and the thumb/no-thumb choice is not very important functionality and can still be achieved via Comcode.

Chris, thank you for the response.

The issues exists even if a simple editor is used; in that case, as I can see, Flash is not involved anywhere.

Almost all over the system browsers block popup windows. I thought it might be somehow related to cookies or Composr installation folder path but couldn't find the real cause.

The attachment uploads are done via swfupload, so the actual buttons clicked to choose a file are flash uploads. Ideally browsers would mark events spawning of a clicked foreground Flash applet as user-generated and thus not blocked, but they don't seem to be smart enough.

However, the user can always put in an exception if they use the site heavily enough to care about attachment thumbnail control.