[2.0.14] Rail signal selection_box shifts incorrectly when not on tile edges

[robot256]

When using selection_box to produce an asymmetric selection area, the actual selection box is different than the entered values specifically when the boundaries are not on tile edges. In these cases, the highlight box accuratly reflects the mouse-selectable area, so this is a different bug than viewtopic.php?f=7&t=119778

Example:
With selection_box = {{-1.5, -0.5}, {-0.5, 0.5}} (and selection_box_shift = 0,0), the selection rectangle is shifted by 1.0 tiles as expected. (East orientation shown)

Screenshot 2024-11-03 200332_2.png (62.43 KiB) Viewed 134 times

I then tried to make the box a little smaller than a full tile. I set selection_box = {{-1.35, -0.65}, {-0.35, 0.35}} (selection_box_shift is still 0,0). The total width of the box should be 0.7x0.7 tiles and the center should still be offset by 1.0. But the result I get is the following:

Screenshot 2024-11-03 200834.png (164.7 KiB) Viewed 134 times

The box is still 1.0x1.0 tiles wide, and the center has been shifted by (0.15, 0.15). This happens in all four cardinal directions (the diagonals have other issues as I reported).