[2.x] Side nav positioning offset missing

[YUCLing]

Current Behavior

With AffixedSidebar, the side nav uses position: fixed for side nav's ul to make it fixed on viewport. However, in #3876, the margin-top of ul is moved to the AffixedSidebar's parent element Page-sidebar.

From the top value used below, we can see it only includes the height of header, so the side nav is at a taller position than 1.x.

framework/framework/core/less/common/sideNav.less

Line 66 in 4feb4a3

top: var(--header-height);

Steps to Reproduce

1. Find a page with a side nav using AffixedSidebar
2. Scroll down

Expected Behavior

The missing offset provided by margin-top should be still there.

Screenshots

Environment

• Flarum version: 2.0-dev

Output of php flarum info

No response

Possible Solution

Backport 11fc74d and ea21ef4, which uses position: sticky and adds the missing offset.

Additional Context

No response