Bugzilla

I debugged this a bit, the test page is doing something like,

`a.com/main.html` --- <iframe> ---> `b.com/child.html`  --- <iframe> ---> `a.com/grandchild.html`. And `a.com/grandchild.html`  reference back to the toplevel window (`a.com/main.html`) via window.open.

And the assertion happens when checking the tabgroup between `a.com/grandchild.html` and `a.com/main.html`.
With Fission enabled, we will do process switching for `a.com/grandchild.html` which will be moved from the process of  `b.com/child.html` to the process of `a.com/main.html` , but we always pass 0 for `aSameTabGroupAs` in https://searchfox.org/mozilla-central/rev/652014ca1183c56bc5f04daf01af180d4e50a91c/dom/ipc/BrowserBridgeParent.cpp#85, so `a.com/grandchild.html` ends up not in the same tabgroup as `a.com/main.html`.

I debugged this a bit, the test page is doing something like,

`a.com/main.html` --- <iframe> ---> `b.com/child.html`  --- <iframe> ---> `a.com/grandchild.html`. And `a.com/grandchild.html`  reference back to the toplevel window (`a.com/main.html`) via window.open.

And the assertion happens when checking the tabgroup between `a.com/grandchild.html` and `a.com/main.html`.
With Fission enabled, we will do process switching for `a.com/grandchild.html` which will be moved from the process of  `b.com/child.html` to the process of `a.com/main.html`  and it setup tabgroup from BrowserChild, but we always pass 0 for `aSameTabGroupAs` in https://searchfox.org/mozilla-central/rev/652014ca1183c56bc5f04daf01af180d4e50a91c/dom/ipc/BrowserBridgeParent.cpp#85, so `a.com/grandchild.html` ends up not in the same tabgroup as `a.com/main.html`.