+++ This bug was initially created as a clone of Bug #1587914 +++ It looks like switching to Reader Mode will always involve a process switch with fission, so the tab will be going through an intermediate `about:blank` in between "regular tab with isArticle" and "tab in reader mode". I expect we'll be seeing a lot more of intermediate `about:blank`s in unexpected places with all the process switches in fission, but not sure we can do much about them.
Bug 1587971 Comment 0 Edit History
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Plain mochitests run in a content process, so when one does a `window.open("http://example.com/")`, that ends up in a different content process under fission.
Which means this test needs to inspect internal state of another content process, which is not easy from a plain mochitest. Solutions are changing this to a browser mochitest, an xpcshell test, or just opening the second tab from the same origin (mochi.test) to keep it in the same process.
Alternatives are mostly equivalent, but third is easiest/smallest change.