Menu item weights out of range, reset to -50

Basically the same problem as in #780720: Menu weight not being set properly..
Menu item weights are saved with the same values as the taxonomy term weights, which is not good because values below -50 and above 50 get reset to -50.
Initially after building the menu it is in the right order (the same order as the taxonomy vocabulary), but saving the menu rearranges it alphabetically (since most items have the weight -50).

I'll try to work out some solution to set the weight of top-level items using some variable, which I understand you used to do in earlier D6 versions.