Phenomenon

Clicking Emacs buttons, expanding menus, opening dropdown lists, or even displaying a popup message when the mouse is over a link, will cause my Linux system to freeze. During this freeze, several processes unrelated to Emacs occupy 100% CPU resource, such as python (started by Anki), sunshine, zotero, etc.

Trial and cause

After asking ChatGPT and trying its suggestions, solutions without effects:

disabling compositor in KDE does not help.
disabling desktop effects in KDE does not help.
previous migration from GTK3 to lucid x-toolkit when compiling Emacs does not help.

The final cause is probably the input method fcitx5.

Solution

Create an alias in $HOME/.bashrc:

alias emacs="GTK_IM_MODULE=none QT_IM_MODULE=none XMODIFIERS=@im=none emacs"

In Emacs, use pyim as the input method. toggle-input-method is bound to M-\.

(global-set-key (kbd "M-\\") #'toggle-input-method)

Lessons learnt

Emacs itself is a very reliable and robust system. Errors and bugs often come from its interaction with other libraries and tools, i.e. as a problem source, collaboration or synergy plays much important role than intrinsic issues. Therefore, when using a complex and powerful tool, we had better rely on its own infrastructure and functionalities, with few external dependencies as possible.