A lot of WebCore code assumes that JSGlobalObject instances are the JSDOMGlobalObject subclass. For example, when `incumbentDOMWindow` is traversing JSGlobalObjects in the callstack, and it assumes that these ones are JSDOMWindow. After introducing Shadow Realms, via https://bugs.webkit.org/show_bug.cgi?id=230602, this won't necessarily be true. That is WebCore's assumption that `JSGlobalObject == JSDOMGlobalObject` might no longer be valid. This ticket a is a reminder that we need to investigate this and properly adapt the code if needed
<rdar://problem/84363152>
Yusuke, you raised this initially. Looking at `incumbentDOMWindow`, it doesn't seem to be used in api contexts that the shadow realm has access to. For example, in the derived JSDOMWindow.cpp, `incumbentDOMWindow` is used for `postMessage`, `closeBody`, `focusBody`, etc, which won't be exposed to the shadow realm global object. Can you think of other areas we need to look out for, or perhaps this assumption isn't an issue?
(In reply to Phillip Mates from comment #2) > Yusuke, you raised this initially. Looking at `incumbentDOMWindow`, it > doesn't seem to be used in api contexts that the shadow realm has access to. > For example, in the derived JSDOMWindow.cpp, `incumbentDOMWindow` is used > for `postMessage`, `closeBody`, `focusBody`, etc, which won't be exposed to > the shadow realm global object. > > Can you think of other areas we need to look out for, or perhaps this > assumption isn't an issue? What happens if you pass these functions to ShadowRealm and calling it inside ShadowRealm's user function?
As mentioned on Slack (https://webkit.slack.com/archives/CTV4FGWF4/p1644353301940789) I think we are pretty comfortable not worrying about this, by the usual design of shadow realms, since when we call into parts of the runtime expecting a JSDOMObject on the stack, we will have the incubating realm's global object installed there because of the existing function wrapping; I'm gonna leave this open, for now, though; since this does change when we install web APIs into the shadow realm context.