Let’s say you are looking at a regular-weight /c/ which has its left sidebearing to be equal to the /o/'s. You want to experiment with it narrower, so you copy the layer and move to work on the copy of the layer. You move the contours in, and then want to update the metrics (Ctrl-Cmd-M). But it doesn’t work, I guess because technically there’s not an exactly corresponding layer in the /o/ slot.
I think a layer that is named Regular … [time stamp] should assume that any references in the sidebearing settings should refer to the Regular layer of the letter referred to.
Does that make sense?
[I recognize that a workaround is to edit the Regular and let the time-stamped layer be the backup, and that they are easily switched with the Use as master function.]
I really dislike this idea. Once I copy a layer I usually want it to stay as is for reference. If you need to create multiple working versions of a letter create alternate glyphs.