When spacing a font it is not unusual to change the sidebearing of the wrong character by mistake (if you work with two screens using the preview panel for instance). Normally it is easy to realize the mistake, but sometimes you just don’t pay attention and may en up with your Hs Os, os or ns displaying wrong side bearings that may result in a complete mess in everytyhing you space after that. The later you realize it, the bigger that mess will be.
In order to avoid this, would it be possible to lock the sidebearings of some characters so that you cannot change them by mistake?
You can enter absolute metrics keys: Put ==XX in the LSB and RSB fields. It will not actually lock the values but will tell you by becoming red, and you can easy reset it by updating the metrics. You could write a small script to put the values in there.
This may be a pretty useful workaround. What I was wondering is if it would be possible to program something so that Glyphs does not allow you to change a locked side bearing (something similar to the warnings you get when filling some form fields wrongly). I think a red side bearing value is not enough, as it is quite common to have red values that are actually right, for instance, when you scale a component and the resultant sidebearing is not an integer. You may get a LSB of 49.9 instead of 50 and the value is displayed in red. And again, if you are in a rush and did not noticed the red side bearing, updating the metrics later would only fix the values in that Glyph, but you will have to check again everything you spaced using it as a reference.