Is there a way to add a reduce spacing in general, without changing the actual Metrics? When doing Kerning it would be helpful to see what happens if for instance one applies really tight spacing.
That is really a task for the Adobe Fonts folder:
http://www.glyphsapp.com/tutorials/testing-your-fonts-in-adobe-apps
If you don’t remove overlaps and also turn off autohinting, the (re)export is very quick.
I agree that this would be an important feature for Glyphs to support internally. The fonts folder solution is not satisfactory if you want to efficiently work on kerning. This feature request has come up before – isn’t it faiirly easy to implement?
Actually, this is possible to simulate in Glyphs. If you set up a spaced-out instance and give it the custom parameter
({Filter = "Transformations;LSB:+50.0;RSB:+50.0;";})
you can kern while seeing a spaced-out (or tightened) preview.
Thats actual a great workaround, thanks Tim!
That is on my list for some time. But I didn’t found time to properly implement it yet. The spacing itself is easy. But to find a good place to put the UI is not. I don’t like to add a control for that.
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I’d personally like to see it done with a slider, so tracking can be increased, decreased and then flattened into the glyph sidebearings when the designer is happy with the results. (Sliders especially helpful in balancing the apparent weight of one script against an existing Latin, for example.)
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I absolutely agree. Need to test kerning with capital spacing etc. So one slider for tracking and a slider for line spacing as well please, for a better look at superiors.
I have requested this feature some times in other places, and I actually used it in my youth in the eighties 
The app KernEdit from 1989 (http://www.macintoshrepository.org/2079-kernedit-1-0-8) has a good interface element: a black triangle left, black rectangle, black triangle right. It should be named Tracking instead of Spacing. The icon for Leading could be arrow-up, rectangle, arrow-down.
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One question about the behavior. Would the tracking be stored for the active edit view or for the master/instance?
My first thought was for the edit view, as it’s a kind of preview, rather than part of the font/instance, but I now think it’s better to store it as an instance setting so we don’t need to keep adjusting the tracking in each new edit view, and can have different tracking settings in each instance.
BTW has anyone established the relationship between a font’s sidebearings and InDesign’s tracking? In my current font, to match InDesign’s tracking of +15, I had to add 5 units each to left and right sidebearings in Glyphs.
Are you sure it is not 7, i.e. half the tracking value?
Do you have a UPM of 1000? I believe InDesign calculates its values for a UPM of 1000.
Yes, I was puzzles that adding 7 and 8 to the left and right made things much too far apart. I was experimenting on a line of 10 of the same character, to try and avoid rounding errors. I’ll keep investigating.