Previously with input method framework like ibus or fcitx, they're able to preserve the input method state while switching the windows or applications.

For example:

Previously user is at LibreOffice inputing Chinese and typing in gnome-terminal with English, now if user switch from LibreOffice to gnome-terminal the input method state should transit from Chinese to English automatically.

Current gnome-shell behaviour is the whole desktop share a single input method, user cannot concentrate on typing at all.