In Firefox, The Custom Element gets cloned when calling member function of props, like toUpperCase()

[wbern]

Sandbox: https://codesandbox.io/s/preact-custom-element-forked-mro87

How to reproduce

Try change the attribute value to something else.

You don't even need to render the string, just call size.toLowerCase() in an expression inside the functional component.

The issue

• The Custom Element gets cloned
• The size becomes null for a second and crashes the demo.

Workaround

Do a check if the size prop is null, and do an early return. The temporary null value is what caused the element to be cloned.

With null checking sandbox: https://codesandbox.io/s/preact-custom-element-forked-40vfr?file=/src/Button.tsx

Just try to change the size attribute as usual.