Look at e.path where available (M55+)

go-back-with-backspace/content_script.js

 // Listen for shift-backspace or unmodified backspace and navigate if not in
 // an editable field. We capture the event at the Window to let any handlers
 // or listeners registered on the Document have a chance to handle it first.
 window.addEventListener('keydown', function(e) {
-  // Listening on the Window means the event has no path (see
-  // http://crbug.com/645527), so we'll have to look at the focused (active)
-  // element. This means it will not work properly with shadow DOM.
-  // TODO: Fix behavior with shadow DOM when the above bug is resolved.
+  // Before Chrome 55, listening on the Window means the event has no path,
+  // instead pointing to the Window (see http://crbug.com/645527). In that
+  // case, we have to look at the focused (active) element.
+  // TODO: Switch entirely to e.path once Chrome 55 is launched.
   if (e.key === 'Backspace' &&
       !e.defaultPrevented &&
       !e.altKey &&
       !e.ctrlKey &&
-      !e.metaKey &&
-      !isEditable(document.activeElement)) {
-    e.shiftKey ? window.history.forward(): window.history.back();
-    e.preventDefault();
+      !e.metaKey) {
+    var target = e.path[0];
+    if (target === window)
+      target = document.activeElement;
+    if (!isEditable(target)) {
+      e.shiftKey ? window.history.forward(): window.history.back();
+      e.preventDefault();
+    }
   }
 });