Scale font to a different UPM

When I use the little button next to the UPM field and enter a new UPM that is only a little different from the old one then this does not work. The shapes are not scaled as expected, and the UPM field still shows the old figure. Seems like the whole change is simply rejected? Oh, wait! No, the vertical metrics are indeed changed! While the UPM stays the same.

Is there any reasoning behind this I don’t understand? Shouldn’t this UPM scaling thing simply do what I tell it to do? I don’t see any reason for some inexplicable magic happening when the change is small.

There was a threshold to avoid scaling that wouldn’t do anything. But that threshold was to high. I fixed it.