Fix buggy header shadow

[PegasisForever]

Fix issue: #2488

Explained in this comment: #2488 (comment)

[squidfunk]

Thanks for investigating. So, in the end, it's only a rounding error, right? Does your solution also work with tabs and the announcement bar?

[PegasisForever]

Ya it's just a rounding error, my code works with tabs and announcement bar, tested on the official docs site.

[squidfunk]

Thanks for the PR and your investigation. It's definitely related to rounding errors. However, the fix doesn't solve the issue in its entirety. When I zoom out, the header shadow is still not visible with your fix applied. The problem stems from the fact that getElementSize uses the offsetWidth and offsetHeight properties, while watchElementSize returns DOMRect objects. The former is the effective size as rendered (thus, no floating point values), the latter what has been computed during layout calculation (floating point values).

We definitely need to make those computations consistent, but we can't just use Math.round, because different browsers round differently, so the behavior would not be consistent.

[squidfunk]

Superseded by f43ef69 – I fixed the root cause.