@Georg

The biggest problem is that many programs abused undocumented features, methods, API calls and internal structures. Not only Stuntzi had an access to sources and IBrowse, ListTree etc had their own peculiar way to peek and poke undocumented stuff.

MUI 4 removed lot of internal stuff like external support libraries. List class was streamlined but IIRC it didn't have major incompatibility issues. I know there is a render quirk in AmIRC lists I should look at some day. I know that the in Zune you are going to have major issue because some old software depends on internal structures you can't know about.

Another problem is of course what you mentioned. MUI design is not perfect and documentation is lacking on many post-V19 stuff. However, stuff like TAG_IGNORE was well known in 90s but all this information has disappeared over time.

[ Edited by itix 18.10.2015 - 10:20 ]