`TypeError: Cannot read properties of undefined (reading 'CSSStyleSheet')` when `defaultView` is undefined

[mkaczkowski]

Hi! 👋

Firstly, thanks for your work on this project! 🙂

Today I used patch-package to patch react-resizable-panels@4.0.15 for the project I'm working on.

Problem

When using react-resizable-panels with Vite + React 19, I encountered a runtime error on pointer move:

Uncaught TypeError: Cannot read properties of undefined (reading 'CSSStyleSheet')
    at ae (updateCursorStyle.ts:20:48)
    at HTMLDocument.Le (onDocumentPointerMove.ts:64:7)

In src/utils/dom/updateCursorStyle.ts, the guard checks if (document.defaultView === null) but document.defaultView can be undefined in certain contexts (Vite HMR, SSR hydration). Since undefined !== null, the guard passes and crashes when accessing defaultView.CSSStyleSheet.

This appears related to PR #555 which addressed popup window support, but the fix was incomplete for the undefined case.

Suggested Fix

In src/utils/dom/updateCursorStyle.ts, change:

if (document.defaultView === null) return;

to:

if (!document.defaultView) return;

Here is the diff that solved my problem (applied to dist files):

-  if (e.defaultView === null)
+  if (!e.defaultView)
     return;

[bvaughn]

In src/utils/dom/updateCursorStyle.ts, the guard checks if (document.defaultView === null) but document.defaultView can be undefined in certain contexts (Vite HMR, SSR hydration). Since undefined !== null

According to the mdn documentation, undefined is not a valid value.

In browsers, document.defaultView returns the window object associated with a document, or null if none is available.

This seems like a bug that should be fixed on the Vite side if it's returning the wrong value. I imagine that would cause problems for other packages too.

[bvaughn]

Added a guard in version 4.0.16

---

❤️ → ☕ givebrian.coffee