diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -9715,7 +9715,16 @@
   // This supports sites using cross-screen window management capabilities to
   // retain fullscreen and place a window on another screen.
+
+  // ForSecurityDropFullscreen calls ExitFullscreen on other WebContents, which
+  // can spin a nested message loop (e.g. ::SetWindowPos on Windows). During
+  // this nested loop, this WebContents can be destroyed (e.g. via
+  // window.close() from a fullscreenchange handler). Guard with a WeakPtr.
+  base::WeakPtr<WebContentsImpl> weak_this = weak_factory_.GetWeakPtr();
   ForSecurityDropFullscreen(display_id).RunAndReset();
+  if (!weak_this) {
+    return;
+  }

   delegate_->SetContentsBounds(this, bounds);
 }