Disable Screenshots page action on non-http(s) pages

[jaredhirsch]

(I think this bug may already exist, but I can't find it right now.)

Some about pages, like about:telemetry or about:mozilla, show the page action menu, and the Screenshots item is enabled there, but it shouldn't be.

The webcompat reporter disables itself in such cases like this: (dxr):

  onLocationChange(window) {
    let action = PageActions.actionForID("webcompat-reporter-button");
    let scheme = window.gBrowser.currentURI.scheme;
    let isReportable = ["http", "https"].includes(scheme);
    action.setDisabled(!isReportable, window);
  },

where onLocationChange is attached inside the PageActions.addAction call (dxr):

    PageActions.addAction(new PageActions.Action({
      // ... stuff ...
      onLocationChange: (window) => this.onLocationChange(window)
    }));

We should do something similar. (See BMO bug 1437445 for more discussion.)

[jaredhirsch]

Hmm, if we land #3967, I guess this fix wouldn't work

[ianb]

I think with #3976 this will be fixed or at least better, so let's defer to that change.