diff --git a/apps/web/src/components/preview/PreviewPanelShell.test.ts b/apps/web/src/components/preview/PreviewPanelShell.test.ts
new file mode 100644
index 00000000000..4ac086157a2
--- /dev/null
+++ b/apps/web/src/components/preview/PreviewPanelShell.test.ts
@@ -0,0 +1,13 @@
+import { describe, expect, it } from "vite-plus/test";
+
+import { getPreviewPanelMaxWidth } from "./PreviewPanelShell";
+
+describe("getPreviewPanelMaxWidth", () => {
+  it("allows the panel to use 70% of an ultra-wide viewport without a pixel ceiling", () => {
+    expect(getPreviewPanelMaxWidth(6_000)).toBe(4_200);
+  });
+
+  it("rounds fractional CSS pixels down", () => {
+    expect(getPreviewPanelMaxWidth(2_001)).toBe(1_400);
+  });
+});
diff --git a/apps/web/src/components/preview/PreviewPanelShell.tsx b/apps/web/src/components/preview/PreviewPanelShell.tsx
index 6de828c1e63..a12c23e386b 100644
--- a/apps/web/src/components/preview/PreviewPanelShell.tsx
+++ b/apps/web/src/components/preview/PreviewPanelShell.tsx
@@ -10,12 +10,14 @@ export type PreviewPanelMode = "inline" | "sheet" | "sidebar" | "embedded";
 
 const PREVIEW_PANEL_WIDTH_STORAGE_KEY = "t3code:preview-panel-width";
 const PREVIEW_PANEL_MIN_WIDTH = 360;
-/** Hard ceiling so a wide monitor can't yield a panel that swallows the chat. */
-const PREVIEW_PANEL_MAX_WIDTH_PX = 1400;
-/** Fraction of the viewport allowed; the panel is min(this · vw, MAX_PX). */
+/** Fraction of the viewport allowed, preserving the remaining space for chat. */
 const PREVIEW_PANEL_MAX_WIDTH_FRACTION = 0.7;
 const PREVIEW_PANEL_DEFAULT_WIDTH = 540;
 
+export function getPreviewPanelMaxWidth(viewportWidth: number): number {
+  return Math.floor(viewportWidth * PREVIEW_PANEL_MAX_WIDTH_FRACTION);
+}
+
 /**
  * Shell for the preview panel. In inline mode the panel is user-resizable
  * via a drag handle on the left edge; width persists per browser. In
@@ -82,5 +84,5 @@ function useViewportClampedMaxWidth(): number {
       if (frame !== 0) window.cancelAnimationFrame(frame);
     };
   }, []);
-  return Math.min(PREVIEW_PANEL_MAX_WIDTH_PX, Math.floor(vw * PREVIEW_PANEL_MAX_WIDTH_FRACTION));
+  return getPreviewPanelMaxWidth(vw);
 }