how to select line to trim when it is on top of fill

if you need to select a line to trim and it is everywhere on top of fill, it cannot be selected, whether you are in draft mode or not.
The way to get around this is to make the layer with the fill invisible. Do that by clicking the "eye" icon of the layer with fill.
Of course this does not work if the line to trim and the fill are on the same layer.