When printing while line numbers are displayed, the Geany editor draws a vertical line between the line number and the text on the printout. Currently, Scintilla prints three spaces as a separator between line number and text.
The version of Geany currently under development uses Scintilla's printing API (previous versions did not). The new version of the Geany code cannot determine the extent of the text on the last printed page, and so cannot determine the correct length of the vertical line.
An API which allows choosing between three spaces and vertical line would allow Geany to eliminate its code which attempts to draw the line, and instead call this new API.