Bug 1737085 - Mac browser_TabUnloader.js Nightly as Beta test failure r=spohl

Test-only fix: pref-on tab unloading and memory-pressure notifications when the test is run on beta or release. Tab unloading and memory pressure notifications are limited to Nightly at this time.

Differential Revision: https://phabricator.services.mozilla.com/D129665

browser/modules/test/browser/browser_TabUnloader.js

@@ -128,12 +128,25 @@ const PREF_AUDIO_LOOPBACK = "media.audio_loopback_dev";
 const PREF_VIDEO_LOOPBACK = "media.video_loopback_dev";
 const PREF_FAKE_STREAMS = "media.navigator.streams.fake";
 const PREF_ENABLE_UNLOADER = "browser.tabs.unloadOnLowMemory";
+const PREF_MAC_LOW_MEM_RESPONSE = "browser.lowMemoryResponseMask";
 
 add_task(async function test() {
   registerCleanupFunction(() => {
     Services.prefs.clearUserPref(PREF_ENABLE_UNLOADER);
+    if (AppConstants.platform == "macosx") {
+      Services.prefs.clearUserPref(PREF_MAC_LOW_MEM_RESPONSE);
+    }
   });
   Services.prefs.setBoolPref(PREF_ENABLE_UNLOADER, true);
+
+  // On Mac, tab unloading and memory pressure notifications are limited
+  // to Nightly so force them on for this test for non-Nightly builds. i.e.,
+  // tests on Release and Beta builds. Mac tab unloading and memory pressure
+  // notifications require this pref to be set.
+  if (AppConstants.platform == "macosx") {
+    Services.prefs.setIntPref(PREF_MAC_LOW_MEM_RESPONSE, 3);
+  }
+
   TabUnloader.init();
 
   // Set some WebRTC simulation preferences.