I updated my desktop to this snapshot yesterday.
Now the key composing in emacs-x11 does not work anymore.
(I use "keysym Menu = Multi_key" in .Xmodmap so that I can, e.g., type <Menu> " a to get an a-umlaut, ä).
The mechanism itself does still work in other applications (terminals, vivaldi browser), and also when running "emacs -nw" in an xterm.
If I hit <Menu> in the X version of emacs, I immediately get a notification in the info line "<Multi_key> is undefined".
Emacs itself has not been updated lately, so likely some linked library, but I have no idea how to locate that.
My laptop is still running at 0219, and does not have this issue.