I think this was added in bug 899648. TabParent::RecvIsParentWindowMainWidgetVisible would crash if called as of today (because windows don't QI to nsIDOMWindowUtils and then we'd get a null-deref). But it's never called, because the only consumer of isParentWindowMainWidgetVisible is in nsPrompter.js in openModalWindow, which is only called in the parent process. Can we just remove the IPC bits here and have nsDOMWindowUtils::GetIsParentWindowMainWidgetVisible throw or return false or crash when called in a child process? Note that bug 1476145 will change TabParent to code that doesn't crash, but it's still dead code.

Yeah, we can just remove this (and kill the sync message while we're at it).

Comment on attachment 8994715 [details] Bug 1477343 - Remove PBrowser::IsParentWindowMainWidgetVisible. r=bz Boris Zbarsky [:bz] (no decent commit message means r-) has approved the revision. https://phabricator.services.mozilla.com/D2347

Comment on attachment 8994715 [details] Bug 1477343 - Remove PBrowser::IsParentWindowMainWidgetVisible. r=bz Andrew McCreight [:mccr8] has approved the revision. https://phabricator.services.mozilla.com/D2347

Pushed by mrbkap@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/2b9fac2e27ef Remove PBrowser::IsParentWindowMainWidgetVisible. r=mccr8,bzbarsky