The SelStart property specifies an offset which is the
starting position of the selected text. This property can be used in
conjunction with the SelLength property to select text in the
virtual display.
To convert the cursor position to an offset, multiply the
y-position by the number of columns and add the x-position. The
SelLength property determines the number of characters to copy
from the starting position. Reading the SelText property
returns the text displayed at the selected location.