fix: workaround for mozilla bug

[funnel-mark]

Firefox has a fun bug, described here:

https://bugzilla.mozilla.org/show_bug.cgi?id=548397
window.getComputedStyle() returns null inside an iframe with display: none

This causes react-sortable-hoc to throw TypeError: can't convert null to object

The workaround is to return an array with a single fake mozilla-prefixed style in it if window.getComputedStyle() returns falsy.

[clauderic]

Hey @funnel-mark, thanks for flagging this and providing a detailed PR!

Based on your proposed implementation however, I wonder if there aren't other situations where window.getComputedStyle() might return a falsy value?

[funnel-mark]

As far as I can understand from the documentation on MDN window.getComputedStyle() should either return a CSSStyleDeclaration or throw an error. It's only because of this bug that it returns null, it should never happen otherwise.

[clauderic]

I see. Thanks for the clarification! 👍

[mikhail-polontech]

Since Firefox version 62, window.getComputedStyle() returns a CSSStyleDeclaration object with length 0 (instead of null) inside an iframe with display: none.
See this comment: https://bugzilla.mozilla.org/show_bug.cgi?id=548397#c53.

CSSStyleDeclaration object with length 0 is not a falsy value, so the fix above does not work anymore and vendorPrefix gets an empty string value.