fixed when block palette scroll up a little (#868)
fixed when block palette scroll up a little (#868)

every time the workspace is rerendered.
sclements/scratch-blocks,LLK/scratch-blocks,LLK/scratch-blocks,griffpatch/scratch-blocks,LLK/scratch-blocks,gengshenghong/scratch-blocks,gengshenghong/scratch-blocks,sclements/scratch-blocks,griffpatch/scratch-blocks,sclements/scratch-blocks,LLK/scratch-blocks,gengshenghong/scratch-blocks,gengshenghong/scratch-blocks,sclements/scratch-blocks,sclements/scratch-blocks,griffpatch/scratch-blocks,griffpatch/scratch-blocks,griffpatch/scratch-blocks,LLK/scratch-blocks,gengshenghong/scratch-blocks