Marks on scrollbar: smarter navigation (center the mark vertically)

Description of the new feature/enhancement

Since #12948 we have the long-awaited marks on the scrollbar, along with the possibility to navigate between them.

Unfortunately, the navigation isn't very smart yet. When jumping to the previous/next mark, the window area merely scrolls such that the marked line becomes visible.

It would be much nicer if

• the marked line would end up somewhere near the middle of the window – more often than not, what I'm interested in is the content directly above or below the mark, and this would then be immediately visible.
• the line was highlighted (briefly) after the navigation action – this would make it easy to immediately find the marked line visually.

Proposed technical implementation details (optional)

The »jump to mark« command should scroll the window area such that the target line ends up near the middle of the screen whenever possible. A temporary highlight effect should be used to visually guide the user to the target line.

Start by tracing the existing jump-to-mark command and scrollbar mark navigation behavior. Identify how the target line is brought into view, then verify that navigation centers it when possible and briefly highlights the line afterward.