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diff --git a/devtools/client/inspector/test/browser_inspector_pane-toggle-03.js b/devtools/client/inspector/test/browser_inspector_pane-toggle-03.js
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+++ b/devtools/client/inspector/test/browser_inspector_pane-toggle-03.js
@@ -0,0 +1,60 @@
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Tests that the 3 pane inspector toggle can render the middle and right panels of equal
+// sizes when the original sidebar can be doubled in width and be smaller than half the
+// toolbox's width in the BOTTOM host.
+
+const SIDEBAR_WIDTH = 200;
+
+add_task(async function () {
+  info("Switch to 2 pane inspector to test the 3 pane toggle button behavior");
+  await pushPref("devtools.inspector.three-pane-enabled", false);
+
+  const { inspector } = await openInspectorForURL("about:blank");
+  const { panelDoc: doc } = inspector;
+  const button = doc.querySelector(".sidebar-toggle");
+  const toolboxWidth = doc.getElementById("inspector-splitter-box").clientWidth;
+
+  if (toolboxWidth < 600) {
+    ok(true, "Can't run the full test because the toolbox width is too small.");
+  } else {
+    info("Set the sidebar width to 200px");
+    inspector.splitBox.setState({ width: SIDEBAR_WIDTH });
+
+    info("Click on the toggle button to toggle ON 3 pane inspector");
+    let onRuleViewAdded = inspector.once("ruleview-added");
+    EventUtils.synthesizeMouseAtCenter(
+      button,
+      {},
+      inspector.panelDoc.defaultView
+    );
+    await onRuleViewAdded;
+
+    info("Checking the sizes of the 3 pane inspector");
+    let sidebarWidth = inspector.splitBox.state.width;
+    const sidebarSplitBoxWidth =
+      inspector.sidebarSplitBoxRef.current.state.width;
+    is(sidebarWidth, SIDEBAR_WIDTH * 2, "Got correct main split box width");
+    is(
+      sidebarSplitBoxWidth,
+      SIDEBAR_WIDTH,
+      "Got correct sidebar split box width"
+    );
+
+    info("Click on the toggle button to toggle OFF the 3 pane inspector");
+    onRuleViewAdded = inspector.once("ruleview-added");
+    EventUtils.synthesizeMouseAtCenter(
+      button,
+      {},
+      inspector.panelDoc.defaultView
+    );
+    await onRuleViewAdded;
+
+    info("Checking the sidebar size of the 2 pane inspector");
+    sidebarWidth = inspector.splitBox.state.width;
+    is(sidebarWidth, SIDEBAR_WIDTH, "Got correct sidebar width");
+  }
+});