The BlockSel structure is used by Selsf to specify the left and right boundaries of a text block selection. The values are pixels at the zoom level in which the selection was made.
zpFirst (2 bytes): A signed integer that specifies the physical left boundary of the selection, in pixels. The physical left page margin is at pixel zero.
zpLim (2 bytes): A signed integer that specifies the physical right boundary of the selection, in pixels. zpLim MUST be greater than or equal to zpFirst.