Hello @cinerea0, please copy your system information from Steam (Steam -> Help -> System Information) and put it in a gist, then include a link to the gist in this issue report.
Your log doesn't really give any decent hints besides mentioning libgtk3-nocsd, which has a history of causing random trouble. Can you remove that package from the mix?
Is there a particular reason why you think that wayland (the protocol) is at fault? There isn't a central compositor for Wayland. Each desktop environment gets to build their own compositor how they see fit, so if there is a DE-specific quirk, then it could be specific to hikari. Can you test how Steam behaves with a different desktop environment?
@kisak-valve I have created a gist containing my system info and I have placed a link to it in my initial comment. I have also disabled libgtk3-nocsd, and I've created another gist that contains the terminal output from when Steam starts here: https://gist.github.com/cinerea0/f2b96c2b8c2adc42c947aedfe592fb2e.
I thought that Steam's performance under Wayland was the problem because I'm also experiencing the issue mentioned above where Steam doesn't respond to mouse input correctly. I had incorrectly described this earlier as "clicking on headings in the Steam client such as "STORE" does not result in any response", but after further investigation it's registering input further down than it should be. Here's a screenshot as an example:

This shows that my cursor is selecting "Collections" under the "LIBRARY" heading despite my cursor being further down the screen.
I tested Steam under a different Wayland compositor (wayfire), and neither the problem with games being unable to launch nor the mouse issue were present. Do you see anything in the gists that I posted that indicates that this could be a failure of some setting on my part or a missing package? If not, then you're right that this is probably a hikari issue.
Thanks, https://gist.github.com/cinerea0/76a22362061004559b9dc1efa1a288da#file-steam_info-json-L44 looks like a relevant detail in your system information. If hikari uses wlroots under the hood, then it could share some behavior with sway since we know that sway is also built on top of wlroots.
What's weird is that wayfire, which worked fine, is also built on wlroots. It could be that wayfire implements some kind of extension to wlroots that hikari doesn't.
It's either an issue in Xwayland or Wlroots/Hikari's implementation of it. Obviously the best solution would be to just make the Steam client be a Wayland client but I feel like we are still years away from that.
Nothing extracted yet.
Your system information
Full system information can be found in the following gist: https://gist.github.com/cinerea0/76a22362061004559b9dc1efa1a288da
Please describe your issue in as much detail as possible:
Upon launching a game, I expected that the game window would open. Instead, the game immediately closed itself and produced the following terminal output:
Given that I am also encountering the issue detailed in #4924 where clicking on headings in the Steam client such as "STORE" does not result in any response, I think this is due to my use of a Wayland compositor.
Steps for reproducing this issue: