Clicking on scrollbar ends up dragging elements underneath

[davidbau]

If you drag the scrollbar of the main editing region, the clicks end up going through to elements that are underneath the scrollbar. To repro,

1. create a page with big blocks that go past the right and bottom edges, like this one: http://david.pencilcode.net/edit/about.html#blocks=1
2. then click on the scrollbar where some block is underneath, and drag the scrollbar.
3. observe: hovering highlights the block, and click-drag drags the block.

expect: the click shouldn't go through the scrollbar.