Maybe standardize window.sidebar

[DavidBruant]

Telemetry data shows that window.sidebar is accessed by about 1.62% of page loads, that's too high to remove.

https://bugzilla.mozilla.org/show_bug.cgi?id=862147#c47
https://bugzilla.mozilla.org/show_bug.cgi?id=1428302

[annevk]

Do other browsers support it?

Also, I suspect you might get that high a number simply due to enumeration. It's quite hard to measure actual usage of such things on the global object. What does the sidebar getter return?

[annevk]

Undefined in all other browsers, so at best this would be restricted to navigator compatibility mode being Gecko.