fuck this shit bro (add tabs.onReplaced notification)

browser/base/content/tabbrowser.xml

@@ -2920,6 +2920,18 @@
             // Finish tearing down the tab that's going away.
             remoteBrowser._endRemoveTab(aOtherTab);
 
+            // Notify WebExtensions when one tab is replaced by another. Tab
+            // adoption uses this same swap internally, but represents a move
+            // and is marked below so it does not generate a replacement event.
+            if (!aOurTab._adoptingTab) {
+              let event = new CustomEvent("TabReplaced", {
+                bubbles: true,
+                detail: {addedTab: aOurTab, removedTab: aOtherTab},
+              });
+              aOurTab.dispatchEvent(event);
+            }
+            delete aOurTab._adoptingTab;
+
             if (isBusy)
               this.setTabTitleLoading(aOurTab);
             else
@@ -3359,6 +3371,7 @@
             params.userContextId = aTab.getAttribute("usercontextid");
           }
           let newTab = this.addTab("about:blank", params);
+          newTab._adoptingTab = aTab;
           let newBrowser = this.getBrowserForTab(newTab);
           let newURL = aTab.linkedBrowser.currentURI.spec;
 

browser/components/webextensions/ext-tabs.js

@@ -103,6 +103,7 @@ let tabListener = {
 
     AllWindowEvents.addListener("TabClose", this);
     AllWindowEvents.addListener("TabOpen", this);
+    AllWindowEvents.addListener("TabReplaced", this);
     WindowListManager.addOpenListener(this.handleWindowOpen);
     WindowListManager.addCloseListener(this.handleWindowClose);
 
@@ -138,6 +139,10 @@ let tabListener = {
           this.emitRemoved(tab, false);
         }
         break;
+
+      case "TabReplaced":
+        this.emitReplaced(event.detail.addedTab, event.detail.removedTab);
+        break;
     }
   },
 
@@ -220,6 +225,13 @@ let tabListener = {
     }, Ci.nsIThread.DISPATCH_NORMAL);
   },
 
+  emitReplaced(addedTab, removedTab) {
+    this.emit("tab-replaced", {
+      addedTabId: TabManager.getId(addedTab),
+      removedTabId: TabManager.getId(removedTab),
+    });
+  },
+
   tabReadyInitialized: false,
   tabReadyPromises: new WeakMap(),
   initializingTabs: new WeakSet(),
@@ -347,7 +359,15 @@ extensions.registerSchemaAPI("tabs", "addon_parent", context => {
         };
       }).api(),
 
-      onReplaced: ignoreEvent(context, "tabs.onReplaced"),
+      onReplaced: new EventManager(context, "tabs.onReplaced", fire => {
+        let listener = (eventName, event) => {
+          fire(event.addedTabId, event.removedTabId);
+        };
+        tabListener.on("tab-replaced", listener);
+        return () => {
+          tabListener.off("tab-replaced", listener);
+        };
+      }).api(),
 
       onMoved: new EventManager(context, "tabs.onMoved", fire => {
         // There are certain circumstances where we need to ignore a move event.