Versions:

• Arch Linux, up to date as of 2022-04-22, on AMD hardware with Mesa 22.0.1
• KDE Plasma Desktop, in either X11 or Wayland mode
• Do not force any specific video driver (SDL uses X11, possibly via Xwayland)
• Good configuration: Portal 2 launched under Steam public beta, without using "Steam Linux Runtime"; Steam Runtime 'scout' v0.20220315.0; no 32-bit SDL installed system-wide; Portal 2 uses 32-bit SDL 2.0.20 (libSDL2-2.0.so.0.18.2) from the scout runtime
• Bad configuration: replace the scout runtime with one that includes SDL 2.0.22rc2 (note for @slouken and other Valve people: this is the steam-runtime.tar.xz from scout internal build smcv-task106-0.20220422.0, please ask me for a copy if you are having trouble reproducing the issue)

Steps to reproduce:

• Load Portal 2
• Load a saved game (in case it matters, mine is shortly after leaving the cell from Portal 1, before you get the portal gun)
• Move the mouse horizontally at a constant moderate-to-fast speed

Expected result:

• Camera direction changes smoothly

Actual result:

• In the good configurations (with SDL 2.0.20), the camera direction changes smoothly
• When moving the mouse slowly, the camera direction changes smoothly
• When moving the mouse at a moderate-to-fast speed with 2.0.22rc2, the camera periodically snaps back to a previous position

Additional notes: I don't know whether this would be considered to be a bug in SDL or whether Portal 2's engine is known to be doing something wrong here, but either way it's a regression triggered by SDL 2.0.22.