Toggling masters while editing sidebearing values

Glyphs
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Hello!

I’ve encountered a problem when working on metrics and toggling between masters. I haven’t found any topics already posted about.

I’m working on a typeface that has two masters (Light and Black). Let’s say I’m working on the metrics of the letter E:

  • on the Light master, the RSB value is ==H-49
  • on the Black master, the RSB value is =H-42

If I’m on the Light master, and I click the RSB value so ==H-49 becomes editable, and then I switch to the other master (by clicking the button at the top-left of the window), the RSB value in the Black master gets overwritten? (That is to say, the RSB value for the Black master is no longer =H-42 but instead ==H-49).

Is this a bug, or am I working improperly? Is it a bad idea to switch between masters to compare spacing values within them?

Here are some screengrabs to show the steps visually:

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It doesn’t make sense to use metrics keys for that. Only use it for shapes that are at least similar. Maybe for the O and D and add a small correction because of the slightly different curvature.

I have a look at the master switching.