fixes dragging failure in blocks palette
fixes dragging failure in blocks palette

as said in issue #873, which in introduced by commit 06fa507.

The change is using workspace.scroll instead of viewTop to calculate the position of the scrollbar.
Because the top of the view and the scrollbar position are two different things.

gengshenghong/scratch-blocks,sclements/scratch-blocks,sclements/scratch-blocks,LLK/scratch-blocks,griffpatch/scratch-blocks,sclements/scratch-blocks,LLK/scratch-blocks,griffpatch/scratch-blocks,griffpatch/scratch-blocks,gengshenghong/scratch-blocks,gengshenghong/scratch-blocks,LLK/scratch-blocks,griffpatch/scratch-blocks,LLK/scratch-blocks,sclements/scratch-blocks,griffpatch/scratch-blocks,gengshenghong/scratch-blocks,gengshenghong/scratch-blocks,LLK/scratch-blocks,sclements/scratch-blocks