This was added in PR 4516 specifically for Safari on iOS devices, but according to MDN it should no longer be necessary now; see https://developer.mozilla.org/en-US/docs/Web/CSS/-webkit-overflow-scrolling#browser_compatibility
According to the MDN compatibility data, this CSS feature:
- Was never implemented anywhere *except* for Safari on iOS.
- Was never standardized and thus never existed in an *unprefixed* version.
- Has now been removed, starting with Safari version 13.
Given that [the FAQ](https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#faq-support) already lists Safari as "Mostly" supported, and that the default viewer is written primarily for Mozilla Firefox, it ought to be fine to remove these CSS rules now.
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