Basically, we'll need to do https://phabricator.services.mozilla.com/D9016 in https://dxr.mozilla.org/mozilla-central/rev/c291143e24019097d087f9307e59b49facaf90cb/mobile/android/chrome/geckoview/GeckoViewContent.js#257-258 per https://phabricator.services.mozilla.com/D9016#220915 What's the application to test GeckoView? How do I build it?

Maybe I'm misunderstanding this bug, but doesn't feel like XBL implementation related.

In-tree, GeckoView is used for PWAs and custom tabs in Fennec, and there's also the GeckoView Example app that's accessible if you switch the build configuration in Android Studio (or run something like ./mach gradle geckoview_example:installLocalWithGeckoBinariesMinApi21Debug). Neither of those implements any real context menus, although you can still catch and check the event handling in GeckoViewContent.js itself. For custom tabs, you can e.g. use something like this (https://github.com/walkingice/CustomTabsLauncher) if you don't know any other app that uses them, whereas for the Geckoview Example app you need to disable e10s from the three dot menu so you can debug GeckoViewContent.js from the WebIDE.

Tim do you want to take this bug? (Does comment 4 answer what you need?)

I don't have an Android environment with me, so it would be more efficient if someone else could do it.