editor: Preserve scroll position when jumping from multibuffer (#9921)

This is a best-effort attempt, as the target offset from the top is just
an estimate; furthermore, this does not account for things like project
search header (which adds a bit of vertical offset by itself and is
removed once we jump into a buffer), but it still should improve the
situation quite a bit.

Fixes: #5296

Release Notes:

- Improved target selection when jumping from multibuffer; final
position in the buffer should more closely match the original position
of the cursor in the multibuffer.