--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -8567,16 +8567,19 @@ nsDocument::DispatchPageTransition(Event
     return;
   }
 
   PageTransitionEventInit init;
   init.mBubbles = true;
   init.mCancelable = true;
   init.mPersisted = aPersisted;
 
+  nsDocShell* docShell = mDocumentContainer.get();
+  init.mInFrameSwap = docShell && docShell->InFrameSwap();
+
   RefPtr<PageTransitionEvent> event =
     PageTransitionEvent::Constructor(this, aType, init);
 
   event->SetTrusted(true);
   event->SetTarget(this);
   EventDispatcher::DispatchDOMEvent(aDispatchTarget, nullptr, event,
                                     nullptr, nullptr);
 }

--- a/dom/base/test/chrome/window_swapFrameLoaders.xul
+++ b/dom/base/test/chrome/window_swapFrameLoaders.xul
@@ -116,16 +116,19 @@ Test swapFrameLoaders with different fra
           });
           addMessageListener("check-browser-api", function() {
             let exists = "api" in this;
             sendAsyncMessage("check-browser-api", {
               exists,
               running: exists && !this.api._shuttingDown,
             });
           });
+          addEventListener("pagehide", function({ inFrameSwap }) {
+            sendAsyncMessage("pagehide", inFrameSwap);
+          });
         }`;
       }
 
       // Load frame script into each frame
       {
         let mmA = frameA.frameLoader.messageManager;
         let mmB = frameB.frameLoader.messageManager;
 
@@ -164,19 +167,27 @@ Test swapFrameLoaders with different fra
         }
       }
 
       // Ping after swap using message managers acquired before
       {
         let mmA = frameA.frameLoader.messageManager;
         let mmB = frameB.frameLoader.messageManager;
 
+        let pagehideA = once(mmA, "pagehide");
+        let pagehideB = once(mmB, "pagehide");
+
         info("swapFrameLoaders");
         frameA.swapFrameLoaders(frameB);
 
+        let [ { data: inFrameSwapA } ] = await pagehideA;
+        ok(inFrameSwapA, "Frame A got pagehide with inFrameSwap: true");
+        let [ { data: inFrameSwapB } ] = await pagehideB;
+        ok(inFrameSwapB, "Frame B got pagehide with inFrameSwap: true");
+
         let inflightA = once(mmA, "pong");
         let inflightB = once(mmB, "pong");
 
         info("Ping message manager for frame A");
         mmA.sendAsyncMessage("ping");
         let [ { data: pongA } ] = await inflightA;
         is(pongA, "B", "Frame A message manager acquired before swap gets reply B after swap");
 

--- a/dom/webidl/PageTransitionEvent.webidl
+++ b/dom/webidl/PageTransitionEvent.webidl
@@ -14,14 +14,19 @@
 interface PageTransitionEvent : Event
 {
   /**
    * Set to true if the document has been or will be persisted across
    * firing of the event.  For example, if a document is being cached in
    * session history, |persisted| is true for the PageHide event.
    */
   readonly attribute boolean persisted;
+
+  // Whether the document is in the middle of a frame swap.
+  [ChromeOnly]
+  readonly attribute boolean inFrameSwap;
 };
 
 dictionary PageTransitionEventInit : EventInit
 {
   boolean persisted = false;
+  boolean inFrameSwap = false;
 };