8383906: Target highlight in member details is too aggressive

[hns]

Please review a change to improve the target highlight for javadoc member details sections.

Instead of highlighting the member header by changing its background color, we now add a subtle accent bar to the left side of the header. The bar is implemented as box-shadow, is just outside the header element and does not cause a layout shift. It uses the theme's link color with an alpha of 70%, which seems to work well in both themes.

The accent bar is wide enough to be noticeable, but nowhere near as distractive as the old highlight, and brings our documentation more in line with modern documentation/UI patterns.

I also slightly increased the scroll-margin-top property of elements to position them a bit further from the navigation bar when targeted by id.

Sample docs for the java.lang package are available here.

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Issue

• JDK-8383906: Target highlight in member details is too aggressive (Enhancement - P4)

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