Under some conditions Emacs will try to resize a frame if its not fullscreen or maximized.
default-text-scale, but I don't think its the only one).
This sends window-manager window (aka Emacs frame) resize commands, which get ignored, making the frame flicker when scaling fonts for eg.
toggle-frame-maximized solves this until the window manager moves the frame which resets Emacs internal state.
(add-to-list 'default-frame-alist '(fullscreen . maximized))
Which works at startup until the frame is moved.
How to configure Emacs so it's always maximized, or so that plugins don't think it's possible to resize the frame?
Tested with spectrwm and notion window managers.