Bug 1915562 - For the SCK backend, Add a pref to allow enumerating displays with the CG backend. r=padenot

Differential Revision: https://phabricator.services.mozilla.com/D220565

dom/media/systemservices/video_engine/desktop_capture_impl.cc

@@ -112,8 +112,15 @@ std::shared_ptr<VideoCaptureModule::DeviceInfo>
 DesktopCaptureImpl::CreateDeviceInfo(const int32_t aId,
                                      const CaptureDeviceType aType) {
   if (aType == CaptureDeviceType::Screen) {
-    return CreateDesktopDeviceInfo(
-        aId, CreateScreenCaptureInfo(CreateDesktopCaptureOptions()));
+    auto options = CreateDesktopCaptureOptions();
+#ifdef XP_MACOSX
+    options.set_allow_sck_capturer(
+        mozilla::StaticPrefs::
+            media_getdisplaymedia_screencapturekit_enabled_AtStartup() &&
+        mozilla::StaticPrefs::
+            media_getdisplaymedia_screencapturekit_enumeration_enabled_AtStartup());
+#endif
+    return CreateDesktopDeviceInfo(aId, CreateScreenCaptureInfo(options));
   }
   if (aType == CaptureDeviceType::Window) {
     return CreateDesktopDeviceInfo(

modules/libpref/init/StaticPrefList.yaml

@@ -11389,6 +11389,16 @@
     type: bool
     value: false
     mirror: once
+
+# Use the libwebrtc ScreenCaptureKit desktop capture backend on Mac for screen
+# enumeration when enabled.
+# When this is false and the backend is used (see above), the older CoreGraphics
+# backend is used to enumerate CGDirectDisplayIDs, which the ScreenCaptureKit
+# backend understand as well.
+-   name: media.getdisplaymedia.screencapturekit.enumeration.enabled
+    type: bool
+    value: false
+    mirror: once
 #endif
 
 # Turn off any cameras (but not mics) while in the background. This is desirable