Bug 1846141 - Don't propagate zoom on replacement for site-specific zoom. r=nika

As otherwise it can race with the front-end setting the right zoom.

This is a bit sketchy, I'm open to other potential solutions?

But we don't have access to per-site zoom prefs in Gecko, so...

Depends on D184922

Differential Revision: https://phabricator.services.mozilla.com/D184923

docshell/base/CanonicalBrowsingContext.cpp

@@ -331,8 +331,13 @@ void CanonicalBrowsingContext::ReplacedBy(
   txn.SetDisplayMode(GetDisplayMode());
   txn.SetForceDesktopViewport(GetForceDesktopViewport());
   txn.SetIsUnderHiddenEmbedderElement(GetIsUnderHiddenEmbedderElement());
-  txn.SetFullZoom(GetFullZoom());
-  txn.SetTextZoom(GetTextZoom());
+
+  // When using site-specific zoom, we let the front-end manage it, otherwise it
+  // can cause weirdness like bug 1846141.
+  if (!StaticPrefs::browser_zoom_siteSpecific()) {
+    txn.SetFullZoom(GetFullZoom());
+    txn.SetTextZoom(GetTextZoom());
+  }
 
   // Propagate the default load flags so that the TRR mode flags are forwarded
   // to the new browsing context. See bug 1828643.

layout/base/tests/test_zoom_restore_bfcache.html

@@ -120,8 +120,8 @@ function handlePageShow(persisted) {
   }
 }
 
-// If Fission is disabled, the pref is no-op.
-SpecialPowers.pushPrefEnv({set: [["fission.bfcacheInParent", true]]}, () => {
+// We want to test Gecko's zoom handling, not the front-end's.
+SpecialPowers.pushPrefEnv({set: [["browser.zoom.siteSpecific", false]]}, () => {
   window.open('file_zoom_restore_bfcache.html', '_blank', 'noopener');
 });
 </script>