Your system information

• Steam client version (build number or date): beta 1644966673, 2022-02-15
• Distribution (e.g. Ubuntu): any host distribution, running the Flatpak app from Flathub (I tested Fedora 35, but it shouldn't matter)
• Opted into Steam client beta?: yes
• Have you checked for system updates?: yes

Steps for reproducing this issue:

1. Have the unofficial Steam Flatpak app from Flathub
2. Opt-in to the beta branch of Steam
3. Do not opt-in to the beta branch of the Flatpak app, com.valvesoftware.Steam//beta (it has a workaround for this issue)
4. Run Steam

Please describe your issue in as much detail as possible:

The steamwebhelper process that provides most of Steam's UI crashes repeatedly, resulting in Steam's main window (the Library, Store, etc.) being blank rectangles (usually black).

The main Steam process repeatedly logs:

steamwebhelper.sh[...]: Runtime for steamwebhelper: defaulting to .../ubuntu12_64/steam-runtime-heavy
src/steamexe/main.cpp (253) : Assertion Failed: reaping pid: ... -- steam

and also

src/common/html/chrome_ipc_client.cpp (410) : m_hCEFHandle == INVALID_PROCESS_HANDLE

~/.var/app/com.valvesoftware.Steam/.local/share/Steam/logs/steamwebhelper.log contains:

...:FATAL:zygote_host_impl_linux.cc(116)] No usable sandbox! Update your kernel or see https://chromium.googlesource.com/chromium/src/+/master/docs/linux/suid_sandbox_development.md for more information on developing with the SUID sandbox. If you want to live dangerously and need an immediate workaround, you can try using --no-sandbox.

as well as some noise related to Steam's crash-reporting mechanism.

~/.var/app/com.valvesoftware.Steam/.local/share/Steam/logs/cef_log.txt has the same message as in ~/.var/app/com.valvesoftware.Steam/.local/share/Steam/logs/steamwebhelper.log.

This was previously reported in https://github.com/ValveSoftware/steam-for-linux/issues/8373 as the third of three possible root causes for symptoms like these.

Workarounds

Choose any one of these:

• Run Steam with the -no-cef-sandbox command-line option (flatpak run com.valvesoftware.Steam -no-cef-sandbox)
• Use the beta branch of the Flatpak app, com.valvesoftware.Steam//beta, until this is resolved
• Run the Steam general-availability branch instead of the beta until this is resolved
  • You can still use Steam → Settings to opt out from the beta, even though most of the UI is crashing
• Set the Steam user interface to Compact mode
• Set the Steam user interface to Big Picture mode

This is not necessarily the same bug you are seeing

Any bug that results in the steamwebhelper crashing will result in the same symptom where the main window is mostly black. This does not mean that the root cause is the same as the one described here.

This particular issue is specific to the unofficial Flatpak app. It is not applicable to native Steam packages such as the official one provided by Valve or the unofficial packages provided by Arch, Debian, Ubuntu, RPMFusion, etc. that do not run in a Flatpak sandbox. If you are using one of those native Steam packages, and you are seeing similar symptoms, then you must be seeing them for a different reason.