diff --git a/packages/app/e2e/browser/mermaid-fullscreen.spec.ts b/packages/app/e2e/browser/mermaid-fullscreen.spec.ts
index 27eacf1b7b..4b458c4d03 100644
--- a/packages/app/e2e/browser/mermaid-fullscreen.spec.ts
+++ b/packages/app/e2e/browser/mermaid-fullscreen.spec.ts
@@ -1,7 +1,8 @@
-import { expect } from "@playwright/test";
 import { test } from "../support/fixtures";
 import {
   closeDiagramFullscreen,
+  expectInlineDiagramFitsViewport,
+  toggleDiagramSourceAndVerifyControls,
   closeDiagramFullscreenFromOutside,
   expectCompletedDiagram,
   expectFullscreenDiagram,
@@ -43,9 +44,7 @@ test("bounds a tall inline diagram", async ({ page }) => {
     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 expectInlineDiagramFitsViewport(page);
     await openDiagramFullscreen(page);
     await expectFullscreenDiagram(page, ["N0", "N30"]);
     await closeDiagramFullscreen(page);
@@ -73,35 +72,8 @@ 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("Source toggles preserve the diagram and exclude hidden controls", async () => {
+      await toggleDiagramSourceAndVerifyControls(page);
     });
 
     await test.step("The toolbar opens the diagram fullscreen", async () => {
diff --git a/packages/app/e2e/support/helpers/diagram.ts b/packages/app/e2e/support/helpers/diagram.ts
index 82fd745de1..48ecc60086 100644
--- a/packages/app/e2e/support/helpers/diagram.ts
+++ b/packages/app/e2e/support/helpers/diagram.ts
@@ -148,3 +148,59 @@ export async function expectFullscreenDiagram(
 export async function expectFullscreenDiagramClosed(page: Page): Promise<void> {
   await expect(page.getByTestId(FULLSCREEN_VIEWPORT)).toHaveCount(0);
 }
+
+/** Tall diagrams stay within the screen so their controls remain reachable. */
+export async function expectInlineDiagramFitsViewport(page: Page): Promise<void> {
+  const viewport = page.getByTestId("mermaid-viewport").last();
+  const box = await viewport.boundingBox();
+  expect(box, "the inline diagram has measurable layout").not.toBeNull();
+  const screenHeight = page.viewportSize()!.height;
+  expect(box!.height, "the inline diagram fits within the screen").toBeLessThan(screenHeight);
+  const contentHeight = await diagramSvg(page, "mermaid-viewport").evaluate(
+    (svg) => (svg as SVGSVGElement).viewBox.baseVal.height,
+  );
+  expect(box!.height, "a tall diagram is bounded rather than expanding the chat").toBeLessThan(
+    contentHeight,
+  );
+}
+
+/** Exercise pointer toggles and the controls encountered along that same journey. */
+export async function toggleDiagramSourceAndVerifyControls(page: Page): Promise<void> {
+  const viewport = page.getByTestId("mermaid-viewport").last();
+  const iframe = await viewport.locator("iframe").elementHandle();
+  for (let iteration = 0; iteration < 3; iteration++) {
+    await viewport.hover();
+    await page.getByRole("button", { name: "View source" }).last().click();
+    const viewDiagram = page.getByRole("button", { name: "View diagram" }).last();
+    await viewDiagram.click({ timeout: 10_000 });
+    await expect(viewDiagram).toHaveCount(0);
+    await expectFullscreenDiagramClosed(page);
+    expect(
+      await viewport.locator("iframe").evaluate((node, original) => node === original, iframe),
+      "switching views preserves the rendered diagram",
+    ).toBe(true);
+  }
+  await viewport.hover();
+  await page.getByRole("button", { name: "View source" }).last().click();
+  const hiddenFullscreen = viewport.getByRole("button", {
+    name: "Fullscreen",
+    includeHidden: true,
+  });
+  await hiddenFullscreen.focus();
+  await expect(hiddenFullscreen).not.toBeFocused();
+  await page.getByRole("button", { name: "View diagram" }).last().click();
+  await viewport.hover();
+  const fullscreen = viewport.getByRole("button", { name: "Fullscreen", exact: true });
+  const box = await viewport.boundingBox();
+  const control = await fullscreen.boundingBox();
+  expect(box).not.toBeNull();
+  expect(control).not.toBeNull();
+  expect(control!.y, "the toolbar starts inside the preview").toBeGreaterThanOrEqual(box!.y);
+  expect(control!.y + control!.height, "the toolbar fits inside the preview").toBeLessThanOrEqual(
+    box!.y + box!.height,
+  );
+  await viewport.getByRole("button", { name: "Zoom in" }).click();
+  await expect(viewport.getByRole("button", { name: "Reset view" })).toBeEnabled();
+  await viewport.getByRole("button", { name: "Reset view" }).click();
+  await expect(viewport.getByRole("button", { name: "Reset view" })).toBeDisabled();
+}