What settings control line selection? Perhaps layer settings.
On some layers, I cannot select a line with the pick tool. Instead I can only pick it by drawing a left and up window.
More commonly, the line in a layer can be picked by clicking near it or on it.
It makes choosing lines to trim, and for hatching polygons complicated to impossible.
The problematic lines were moved to the top to ensure they were not under others.
It turns out I cannot pick the line when it is on top of an underlying locked Hatch, and has nothing to do with a particular layer.
It can be picked in an area were nothing is under it.
By turning the underlying hatch to invisible the line can be picked as usual.
Good to know what you have found out. This behaviour is the same as with an image in the background. Always the image is selected, even if its layer is locked. You have to switch it to invisible. But often you need to see it to know what you are doing. Selection by window works, too.