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diff --git a/devtools/server/tests/chrome/inactive-property-helper/positioned.mjs b/devtools/server/tests/chrome/inactive-property-helper/positioned.mjs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// InactivePropertyHelper positioned elements test cases.
+
+// These are the properties we care about, those that are inactive when the element isn't
+// positioned.
+const PROPERTIES = [
+  { property: "z-index", value: "2" },
+  { property: "top", value: "20px" },
+  { property: "right", value: "20px" },
+  { property: "bottom", value: "20px" },
+  { property: "left", value: "20px" },
+];
+
+// These are all of the different position values and whether the above properties are
+// active or not for each.
+const POSITIONS = [
+  { position: "unset", isActive: false },
+  { position: "initial", isActive: false },
+  { position: "inherit", isActive: false },
+  { position: "static", isActive: false },
+  { position: "absolute", isActive: true },
+  { position: "relative", isActive: true },
+  { position: "fixed", isActive: true },
+  { position: "sticky", isActive: true },
+];
+
+function makeTestCase(property, value, position, isActive) {
+  return {
+    info: `${property} is ${
+      isActive ? "" : "in"
+    }active when position is ${position}`,
+    property,
+    tagName: "div",
+    rules: [`div { ${property}: ${value}; position: ${position}; }`],
+    isActive,
+  };
+}
+
+// Make the test cases for all the combinations of PROPERTIES and POSITIONS
+const mainTests = [];
+
+for (const { property, value } of PROPERTIES) {
+  for (const { position, isActive } of POSITIONS) {
+    mainTests.push(makeTestCase(property, value, position, isActive));
+  }
+}
+
+// Add a few test cases to check that z-index actually works inside grids and flexboxes.
+mainTests.push({
+  info: "z-index is active even on unpositioned elements if they are grid items",
+  property: "z-index",
+  createTestElement: rootNode => {
+    const container = document.createElement("div");
+    const element = document.createElement("span");
+    container.append(element);
+    rootNode.append(container);
+    return element;
+  },
+  rules: ["div { display: grid; }", "span { z-index: 3; }"],
+  ruleIndex: 1,
+  isActive: true,
+});
+
+mainTests.push({
+  info: "z-index is active even on unpositioned elements if they are flex items",
+  property: "z-index",
+  createTestElement: rootNode => {
+    const container = document.createElement("div");
+    const element = document.createElement("span");
+    container.append(element);
+    rootNode.append(container);
+    return element;
+  },
+  rules: ["div { display: flex; }", "span { z-index: 3; }"],
+  ruleIndex: 1,
+  isActive: true,
+});
+
+export default mainTests;