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diff --git a/devtools/client/inspector/grids/test/browser_grids_grid-list-on-mutation-element-added.js b/devtools/client/inspector/grids/test/browser_grids_grid-list-on-mutation-element-added.js
new file mode 100644
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+++ b/devtools/client/inspector/grids/test/browser_grids_grid-list-on-mutation-element-added.js
@@ -0,0 +1,100 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Tests that the grid list updates when a new grid container is added to the page.
+
+const TEST_URI = `
+  <style type='text/css'>
+    .grid {
+      display: grid;
+    }
+  </style>
+  <div id="grid1" class="grid">
+    <div class="cell1">cell1</div>
+    <div class="cell2">cell2</div>
+  </div>
+  <div id="grid2">
+    <div class="cell1">cell1</div>
+    <div class="cell2">cell2</div>
+  </div>
+`;
+
+add_task(async function () {
+  await pushPref("devtools.gridinspector.maxHighlighters", 1);
+  await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  const { inspector, gridInspector } = await openLayoutView();
+  const { document: doc } = gridInspector;
+  const { highlighters, store } = inspector;
+
+  await selectNode("#grid", inspector);
+  const gridList = doc.getElementById("grid-list");
+  const checkbox1 = gridList.children[0].querySelector("input");
+
+  info("Checking the initial state of the Grid Inspector.");
+  is(gridList.childNodes.length, 1, "One grid container is listed.");
+  ok(
+    !highlighters.gridHighlighters.size,
+    "No CSS grid highlighter exists in the highlighters overlay."
+  );
+
+  info("Toggling ON the CSS grid highlighter from the layout panel.");
+  let onHighlighterShown = highlighters.once("grid-highlighter-shown");
+  checkbox1.click();
+  await onHighlighterShown;
+
+  info("Checking the CSS grid highlighter is created.");
+  is(highlighters.gridHighlighters.size, 1, "CSS grid highlighter is shown.");
+
+  info("Adding the #grid2 container in the content page.");
+  const onGridListUpdate = waitUntilState(
+    store,
+    state =>
+      state.grids.length == 2 &&
+      state.grids[0].highlighted &&
+      !state.grids[1].highlighted
+  );
+  SpecialPowers.spawn(gBrowser.selectedBrowser, [], () =>
+    content.document.getElementById("grid2").classList.add("grid")
+  );
+  await onGridListUpdate;
+
+  info("Checking the new Grid Inspector state.");
+  is(gridList.childNodes.length, 2, "Two grid containers are listed.");
+  is(highlighters.gridHighlighters.size, 1, "CSS grid highlighter is shown.");
+
+  const checkbox2 = gridList.children[1].querySelector("input");
+
+  info("Toggling ON the CSS grid highlighter for #grid2.");
+  onHighlighterShown = highlighters.once("grid-highlighter-shown");
+  let onCheckboxChange = waitUntilState(
+    store,
+    state =>
+      state.grids.length == 2 &&
+      !state.grids[0].highlighted &&
+      state.grids[1].highlighted
+  );
+  checkbox2.click();
+  await onHighlighterShown;
+  await onCheckboxChange;
+
+  info("Checking the CSS grid highlighter is still shown.");
+  is(highlighters.gridHighlighters.size, 1, "CSS grid highlighter is shown.");
+
+  info("Toggling OFF the CSS grid highlighter from the layout panel.");
+  const onHighlighterHidden = highlighters.once("grid-highlighter-hidden");
+  onCheckboxChange = waitUntilState(
+    store,
+    state =>
+      state.grids.length == 2 &&
+      !state.grids[0].highlighted &&
+      !state.grids[1].highlighted
+  );
+  checkbox2.click();
+  await onHighlighterHidden;
+  await onCheckboxChange;
+
+  info("Checking the CSS grid highlighter is not shown.");
+  ok(!highlighters.gridHighlighters.size, "No CSS grid highlighter is shown.");
+});