diff --git a/packages/app/e2e/browser/mermaid-fullscreen.spec.ts b/packages/app/e2e/browser/mermaid-fullscreen.spec.ts
index 72b05ed83e..27eacf1b7b 100644
--- a/packages/app/e2e/browser/mermaid-fullscreen.spec.ts
+++ b/packages/app/e2e/browser/mermaid-fullscreen.spec.ts
@@ -1,3 +1,4 @@
+import { expect } from "@playwright/test";
 import { test } from "../support/fixtures";
 import {
   closeDiagramFullscreen,
@@ -21,6 +22,38 @@ const MERMAID_DIAGRAM = [
   "```",
 ].join("\n");
 
+test("bounds a tall inline diagram", async ({ page }) => {
+  test.setTimeout(120_000);
+  const diagram = [
+    "```mermaid",
+    "flowchart TD",
+    ...Array.from({ length: 30 }, (_, i) => `N${i} --> N${i + 1}`),
+    "```",
+  ].join("\n");
+  const agent = await seedMockAgentWorkspace({
+    repoPrefix: "mermaid-tall-",
+    title: "Tall Mermaid",
+    featureValues: {
+      mockStreamingAssistantResponse: diagram,
+      mockStreamingAssistantIntervalMs: 10,
+    },
+  });
+  try {
+    await openAgentRoute(page, agent);
+    await requestDiagram(agent);
+    await waitForDiagramTurnToComplete(agent);
+    await expectCompletedDiagram(page, ["N0", "N30"]);
+    const viewport = page.getByTestId("mermaid-viewport").last();
+    expect((await viewport.boundingBox())!.height).toBe(480);
+    await page.screenshot({ path: test.info().outputPath("tall-preview.png") });
+    await openDiagramFullscreen(page);
+    await expectFullscreenDiagram(page, ["N0", "N30"]);
+    await closeDiagramFullscreen(page);
+  } finally {
+    await agent.cleanup();
+  }
+});
+
 test("opens a Mermaid diagram fullscreen and closes it again", async ({ page }) => {
   test.setTimeout(120_000);
 
@@ -40,6 +73,37 @@ test("opens a Mermaid diagram fullscreen and closes it again", async ({ page })
       await expectCompletedDiagram(page, ["Start", "Ship"]);
     });
 
+    await test.step("Source toggles preserve the iframe and exclude hidden controls", async () => {
+      const viewport = page.getByTestId("mermaid-viewport").last();
+      const iframe = await viewport.locator("iframe").elementHandle();
+      for (let i = 0; i < 3; i++) {
+        await viewport.hover();
+        await page.getByRole("button", { name: "View source" }).last().click();
+        const viewDiagram = page.getByRole("button", { name: "View diagram" }).last();
+        if (i === 0) await page.screenshot({ path: test.info().outputPath("source.png") });
+        await viewDiagram.click();
+        await expect(viewDiagram).toHaveCount(0);
+        if (i === 0) await page.screenshot({ path: test.info().outputPath("diagram.png") });
+        await expect(page.getByTestId("mermaid-fullscreen-viewport")).toHaveCount(0);
+        expect(
+          await viewport.locator("iframe").evaluate((node, original) => node === original, iframe),
+        ).toBe(true);
+      }
+      await viewport.hover();
+      await page.getByRole("button", { name: "View source" }).last().click();
+      await viewport.getByTestId("mermaid-fullscreen").focus();
+      await expect(viewport.getByTestId("mermaid-fullscreen")).not.toBeFocused();
+      await page.getByRole("button", { name: "View diagram" }).last().click();
+      const box = await viewport.boundingBox();
+      expect(box!.height).toBeGreaterThanOrEqual(56);
+      expect(box!.height).toBeLessThan(480);
+      await viewport.hover();
+      await page.getByRole("button", { name: "Zoom in" }).last().click();
+      await expect(page.getByRole("button", { name: "Reset view" }).last()).toBeEnabled();
+      await page.getByRole("button", { name: "Reset view" }).last().click();
+      await expect(page.getByRole("button", { name: "Reset view" }).last()).toBeDisabled();
+    });
+
     await test.step("The toolbar opens the diagram fullscreen", async () => {
       await openDiagramFullscreen(page);
       await expectFullscreenDiagram(page, ["Start", "Review", "Ship"]);
diff --git a/packages/app/src/components/markdown/fence/mermaid/host.web.tsx b/packages/app/src/components/markdown/fence/mermaid/host.web.tsx
index 9370bc90c0..94d6ccc666 100644
--- a/packages/app/src/components/markdown/fence/mermaid/host.web.tsx
+++ b/packages/app/src/components/markdown/fence/mermaid/host.web.tsx
@@ -72,7 +72,7 @@ function MermaidFenceHostImpl({
     () => [
       getDiagramBoxStyle(textStyle),
       containerStyle,
-      { height: runtimeHeight, minHeight: MIN_DIAGRAM_BOX_HEIGHT },
+      { height: Math.min(runtimeHeight, MAX_PREVIEW_HEIGHT), minHeight: MIN_DIAGRAM_BOX_HEIGHT },
     ],
     [runtimeHeight, textStyle],
   );
@@ -111,20 +111,22 @@ function MermaidFenceHostImpl({
           ) : null}
         </View>
       ) : null}
-      <ZoomableViewport
-        accessibilityLabel={t("message.diagram.diagram")}
-        actions={actions}
-        contentSize={diagramSize}
-        style={viewportStyle}
-        testID="mermaid-viewport"
-        wheelActivation="modifier"
-      >
-        <MermaidIframeRuntime
-          request={request}
-          onRendered={handleRendered}
-          onRenderFailed={renderFailed}
-        />
-      </ZoomableViewport>
+      <div inert={!diagramVisible} style={boundaryStyle}>
+        <ZoomableViewport
+          accessibilityLabel={t("message.diagram.diagram")}
+          actions={actions}
+          contentSize={diagramSize}
+          style={viewportStyle}
+          testID="mermaid-viewport"
+          wheelActivation="modifier"
+        >
+          <MermaidIframeRuntime
+            request={request}
+            onRendered={handleRendered}
+            onRenderFailed={renderFailed}
+          />
+        </ZoomableViewport>
+      </div>
       {isFullscreen && visible ? (
         <MermaidFullscreenViewer
           source={visible.source}
@@ -142,6 +144,8 @@ const MEASURING_SIZE = { width: 240, height: 240 };
  * with `overflow: hidden`, so a shorter box leaves the buttons half-clipped and unclickable.
  */
 const MIN_DIAGRAM_BOX_HEIGHT = 56;
+const MAX_PREVIEW_HEIGHT = 480;
+const boundaryStyle: React.CSSProperties = { display: "contents" };
 const sourceContainerStyle: ViewStyle = { position: "relative" };
 /**
  * The viewport's own root is `flex: 1`, so as a flex item it has `flex-basis: 0%` and that basis