Even when you are national-state-level target, there are easier ways to grab the screen.

For local state, it's easier to just install a wireless camera and watch your screen from behind: it leaves no trace on your computer (you may spot it wireless connection, if you lucky). Moreover, they are more interested in your communication devices (your smartphone) than in your desktop.

Foreign states may exploit your notebook builtin "anti-theft" system, Intel Management Engine ("intel" is very good name for a CPU ;-), bugs in NVidia firmware (fonts, OpenGL, etc), bugs in hardware (create a second display to mirror image from primary display to, even when physical display is not attached, for example), etc.

However, I saw that my Firefox window was spied by Chromium window few years ago (I recorded it on Youtube), so this problem in X11 is real.

I am not sure what you saw, but on regular Linux processes of the user can spy on each other anyway. In any case, X had the concept of untrusted clients basically forever but nobody cared to invest even the small amount of work necessary to make it work well because nobody thought it would make a different. That this was later used as a major argument against X convinced me that this is not at all about technology.