Here's patch which adds makes Message::SetSelection call ContainerNeedsUpdate(Update::Selection) when it changes multiple selection so it's actually like the 3.6.3 history said: "Send SCN_UPDATEUI with SC_UPDATE_SELECTION when the application changes multiple selection." and also like the other messages.
I'm not sure if it just should send the update even if it doesn't change multiple selection so I didn't change that.
https://patch-diff.githubusercontent.com/raw/jrsoftware/scintilla/pull/7.diff
https://patch-diff.githubusercontent.com/raw/jrsoftware/scintilla/pull/7.patch
https://github.com/jrsoftware/scintilla/pull/7/files
There are other issues related to this including [#1723].
Related
Bugs:
#1723