[ogurechny]

The distinction probably helped with management of busy or hung processes which didn't process window messages. Window system internals probably yank the non-client messages from the queue, and do the default processing to make window movement and closing with X button work reliably.

[silon42]

Yeah, this was a big problem with OS/2 and why I don't like to use client side decorations on Linux.

[ahartmetz]

It's interesting that someone already tried that long before Wayland and Gnome, with the predictable result...