Juergen, there is some code which is turned off in xforms_helpers.C for the updateWidgetsFromLength methods. I did not turn them on at the time (look at the #if 1) because I wanted to be safe, but it seems to me that they just work (and they are much simpler than the original versions). Could you try them a bit and turn them on if the satisfy you? I remember that I thought at the time they were OK, but I am probably not familiar enough with the old code to be sure that they do the same thing.
Hmm, not that I look at them again, it seem they will only work for plain LyXLenght, but LyXGlueLength. Anyway, I guess a different set of methods should be needed for glue lengths. Comments? JMarc