Map: horizontal grid lines end early if map is not square

If I adjust the size of the map so that it is shorter vertically than it is horizontally then the horizontal yellow lines stop mid-way across the window. I guess when deciding how long the horizontal lines should be, the calculation is using the height of the window instead of the width. The same issue is not present for the vertical lines if the map window is made tall and skinny.

Reproduce the issue by resizing the map window to be wider than it is tall, then locate the map rendering entry point that draws the horizontal and vertical grid lines. Compare the line-length calculations for both directions; done means horizontal lines extend across the full map window in non-square layouts without regressing vertical lines.

Written by the indexing model from the issue text.