I think that the magenta highlight display of the selected convoy added in r10352 is not good enough.
(https://i.imgur.com/HB0qZKl.png)
Zooming is not properly supported, and when zooming in, a split block is displayed as shown in the image. Also, the display position is slightly off.
display_fillbox_wh_clip_rgb(veh_pos.x, veh_pos.y, zoom_in, zoom_in, color_idx_to_rgb(COL_MAGENTA), true);
This change will display magenta pixels to overwrite the existing yellow display.
But I don't know if that is your intention. You could also make it look bigger and more prominent. (´・ω・｀)
Indeed, fixed in r10358