--- a/devtools/client/inspector/rules/test/browser.ini
+++ b/devtools/client/inspector/rules/test/browser.ini
@@ -138,16 +138,17 @@ skip-if = os == "mac" # Bug 1245996 : cl
 [browser_rules_editable-field-focus_01.js]
 [browser_rules_editable-field-focus_02.js]
 [browser_rules_eyedropper.js]
 [browser_rules_filtereditor-appears-on-swatch-click.js]
 [browser_rules_filtereditor-commit-on-ENTER.js]
 [browser_rules_filtereditor-revert-on-ESC.js]
 skip-if = (os == "win" && debug) # bug 963492: win.
 [browser_rules_grid-highlighter-on-navigate.js]
+[browser_rules_grid-highlighter-on-reload.js]
 [browser_rules_grid-toggle_01.js]
 [browser_rules_grid-toggle_02.js]
 [browser_rules_grid-toggle_03.js]
 [browser_rules_guessIndentation.js]
 [browser_rules_inherited-properties_01.js]
 [browser_rules_inherited-properties_02.js]
 [browser_rules_inherited-properties_03.js]
 [browser_rules_inline-source-map.js]

new file mode 100644
--- /dev/null
+++ b/devtools/client/inspector/rules/test/browser_rules_grid-highlighter-on-reload.js
@@ -0,0 +1,53 @@
+/* vim: set ft=javascript ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that a grid highlighter showing grid gaps can be displayed after reloading the
+// page (Bug 1342051).
+
+const TEST_URI = `
+  <style type='text/css'>
+    #grid {
+      display: grid;
+      grid-gap: 10px;
+    }
+  </style>
+  <div id="grid">
+    <div id="cell1">cell1</div>
+    <div id="cell2">cell2</div>
+  </div>
+`;
+
+add_task(function* () {
+  yield addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+
+  info("Check that the grid highlighter can be displayed");
+  yield checkGridHighlighter();
+
+  info("Close the toolbox before reloading the tab");
+  let target = TargetFactory.forTab(gBrowser.selectedTab);
+  yield gDevTools.closeToolbox(target);
+
+  yield refreshTab(gBrowser.selectedTab);
+
+  info("Check that the grid highlighter can be displayed after reloading the page");
+  yield checkGridHighlighter();
+});
+
+function* checkGridHighlighter() {
+  let {inspector, view} = yield openRuleView();
+  let {highlighters} = view;
+
+  yield selectNode("#grid", inspector);
+  let container = getRuleViewProperty(view, "#grid", "display").valueSpan;
+  let gridToggle = container.querySelector(".ruleview-grid");
+
+  info("Toggling ON the CSS grid highlighter from the rule-view.");
+  let onHighlighterShown = highlighters.once("highlighter-shown");
+  gridToggle.click();
+  yield onHighlighterShown;
+
+  ok(highlighters.gridHighlighterShown, "CSS grid highlighter is shown.");
+}

--- a/devtools/server/actors/highlighters/css-grid.js
+++ b/devtools/server/actors/highlighters/css-grid.js
@@ -212,16 +212,21 @@ CssGridHighlighter.prototype = extend(Au
 
     return container;
   },
 
   destroy() {
     this.highlighterEnv.off("navigate", this.onNavigate);
     this.highlighterEnv.off("will-navigate", this.onWillNavigate);
     this.markup.destroy();
+
+    // Clear the pattern cache to avoid dead object exceptions (Bug 1342051).
+    gCachedGridPattern.delete(ROW_KEY);
+    gCachedGridPattern.delete(COLUMN_KEY);
+
     AutoRefreshHighlighter.prototype.destroy.call(this);
   },
 
   getElement(id) {
     return this.markup.getElement(this.ID_CLASS_PREFIX + id);
   },
 
   get ctx() {