Well, I found a way around the weirdness. I tried installing after a reboot and got the same tiny window, but a SECOND click of install on the steam screen brought up the normal installation window.
So I got 7DTD back, but Rust continued to fail as did the next three games I tried.
==> stats_log.txt <==
[2021-11-27 21:38:20] [AppID 251570] Loaded schema from disk.
[2021-11-27 21:38:20] [AppID 251570] Loading stats from disk...
[2021-11-27 21:38:20] [AppID 251570] Loading stats from disk...success
[2021-11-27 21:38:21] [AppID 251570] CAPIJobRequestUserStats - no stats data in server response, we must be up to date
==> appinfo_log.txt <==
[2021-11-27 21:38:22] RequestAppInfoUpdate: AppIDs 251570
[2021-11-27 21:38:22] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:38:22] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
==> configstore_log.txt <==
[2021-11-27 21:38:22] Flushed store 'install' to '/mnt/steam/steam/config/config.vdf'
==> appinfo_log.txt <==
[2021-11-27 21:38:26] RequestAppInfoUpdate: AppIDs 251570
[2021-11-27 21:38:26] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:38:26] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
==> content_log.txt <==
[2021-11-27 21:38:39] AppID 251570 state changed : Update Required,
==> workshop_log.txt <==
[2021-11-27 21:38:39] [AppID 251570] Loaded workshop items in "/mnt/steam/steam/steamapps/workshop" (0 installed, 0 needed)
==> content_log.txt <==
[2021-11-27 21:38:39] AppID 251570 scheduler update : Priority First, not played for 0 seconds, update disabled for 0 seconds
[2021-11-27 21:38:39] AppID 251570 state changed : Update Required,Update Queued,
[2021-11-27 21:38:40] AppID 251570 state changed : Update Required,Update Queued,Update Running,
==> configstore_log.txt <==
[2021-11-27 21:38:40] Flushed store 'userlocal' to '/mnt/steam/steam/userdata/REDACTED/config/localconfig.vdf'
==> content_log.txt <==
[2021-11-27 21:38:40] AppID 251570 update changed : Running,
[2021-11-27 21:38:40] AppID 251570 update changed : Running,Reconfiguring,
[2021-11-27 21:38:40] Created download interface of type 'SteamCache' (7) to host cache1-atl1.steamcontent.com (cache1-atl1.steamcontent.com)
[2021-11-27 21:38:40] Created download interface of type 'SteamCache' (7) to host cache3-lax1.steamcontent.com (cache3-lax1.steamcontent.com)
[2021-11-27 21:38:40] Created download interface of type 'CDN' (2) to host google.cdn.steampipe.steamcontent.com (google.cdn.steampipe.steamcontent.com)
==> shader_log.txt <==
[2021-11-27 21:38:43] Queuing depot manifest 1075016100639355783 (5176029 bytes) for bucket ID 228015, type 7e2ff036a05b1d69/L2:4e72b67faf2c55a81064f0f669542d15 (AppID 251570).
==> configstore_log.txt <==
[2021-11-27 21:38:44] Flushed store 'userlocal' to '/mnt/steam/steam/userdata/REDACTED/config/localconfig.vdf'
==> content_log.txt <==
[2021-11-27 21:38:45] HTTP (SteamCache,15) - cache1-atl1.steamcontent.com (151.205.17.57:80 / 151.205.17.57:80, host: cache1-atl1.steamcontent.com): qn-00201-sltnpastd03-0110104a-1.steam-content-dnld-1.qwilted-cds.cqloud.com/cache1-atl1.steamcontent.com/depot/251578/manifest/1562221519351495492/5/1784501160115002370 - received 200 (OK) HTTP response
[2021-11-27 21:38:46] AppID 251570 update changed : Running,
[2021-11-27 21:38:46] AppID 251570 update changed : Running,Validating,
==> shader_log.txt <==
[2021-11-27 21:38:45] Queuing depot manifest 5655415328003394686 (14432087 bytes) for bucket ID 1541162, type SteamSwarm/G7:VulkanPipelinesV6 (AppID 251570).
==> content_log.txt <==
[2021-11-27 21:38:46] AppID 251570 update changed : Running,
[2021-11-27 21:38:46] AppID 251570 update changed : Running,Preallocating,
[2021-11-27 21:38:47] AppID 251570 update changed : Running,
[2021-11-27 21:38:48] AppID 251570 update started : download 0/5094853104, store 0/0, reuse 0/0, delta 0/0, stage 0/11373702223
[2021-11-27 21:38:48] AppID 251570 state changed : Update Required,Update Queued,Update Running,Update Started,
[2021-11-27 21:38:48] AppID 251570 update changed : Running,Downloading,Staging,
[2021-11-27 21:38:48] Downloading 16004 chunks from depot 251578
[2021-11-27 21:38:49] AppID 251570 scheduler update : Priority User Initiated, not played for 10 seconds, update disabled for 0 seconds
[2021-11-27 21:38:49] HTTP (CDN,35) - google.cdn.steampipe.steamcontent.com (0.0.0.0:0 / 0.0.0.0:0, host: google.cdn.steampipe.steamcontent.com): AuthenticateDepotID (251578) - Success!
==> configstore_log.txt <==
[2021-11-27 21:38:50] Flushed store 'userlocal' to '/mnt/steam/steam/userdata/REDACTED/config/localconfig.vdf'
==> content_log.txt <==
[2021-11-27 21:38:51] AppID 251570 update canceled : Shader Priority (Suspended)
[2021-11-27 21:38:51] AppID 251570 update changed : Running,Downloading,Staging,Stopping,
==> configstore_log.txt <==
[2021-11-27 21:38:53] Flushed store 'install' to '/mnt/steam/steam/config/config.vdf'
==> content_log.txt <==
[2021-11-27 21:38:53] AppID 251570 update changed : Running,Stopping,
[2021-11-27 21:38:53] AppID 251570 update changed : None
[2021-11-27 21:38:53] AppID 251570 state changed : Update Required,Update Queued,Update Started, (Suspended)
[2021-11-27 21:38:53] AppID 251570 state changed : Update Required,Update Started,
[2021-11-27 21:38:53] AppID 251570 scheduler finished : removed from schedule (result Suspended, state 0x40a)
[2021-11-27 21:38:56] AppID 251570 scheduler update : Priority Auto Update, not played for 17 seconds, update disabled for 0 seconds
[2021-11-27 21:38:56] AppID 251570 state changed : Update Required,Update Queued,Update Started,
==> shader_log.txt <==
[2021-11-27 21:38:56] [AppID 251570] Starting Shader Swarm download job.
==> content_log.txt <==
[2021-11-27 21:38:56] Increasing target number of download connections to 4 (rate was 0.000, now 368.972)
[2021-11-27 21:38:56] Current download rate: 368.972 Mbps
[2021-11-27 21:38:56] AppID 251570 state changed : Update Required,Update Queued,Update Running,Update Started,
[2021-11-27 21:38:56] AppID 251570 update changed : Running,
[2021-11-27 21:38:56] AppID 251570 update changed : Running,Reconfiguring,
[2021-11-27 21:38:56] Created download interface of type 'SteamCache' (7) to host cache1-sea1.steamcontent.com (cache1-sea1.steamcontent.com)
[2021-11-27 21:38:59] HTTP (SteamCache,1) - cache1-sea1.steamcontent.com (151.205.17.57:443 / 151.205.17.57:443, host: cache1-sea1.steamcontent.com): qn-00201-sltnpastd03-0110104a-1.steam-content-dnld-1.qwilted-cds.cqloud.com/cache1-sea1.steamcontent.com/depot/251570/manifest/1075016100639355783/5/3789474656651204224 - received 200 (OK) HTTP response
==> shader_log.txt <==
[2021-11-27 21:38:56] [AppID 251570] Target manifest 1075016100639355783 of size 5176029 key 7e2ff036a05b1d69 / L2:4e72b67faf2c55a81064f0f669542d15.
==> content_log.txt <==
[2021-11-27 21:38:59] AppID 251570 update changed : Running,
[2021-11-27 21:38:59] AppID 251570 update changed : Running,Preallocating,
[2021-11-27 21:38:59] AppID 251570 update changed : Running,
[2021-11-27 21:39:00] AppID 251570 update started : download 0/5012864, store 0/0, reuse 0/0, delta 0/0, stage 0/5176029
[2021-11-27 21:39:00] AppID 251570 update changed : Running,Downloading,Staging,
[2021-11-27 21:39:00] Downloading 5 chunks from depot 251570
[2021-11-27 21:39:00] HTTP (CDN,35) - google.cdn.steampipe.steamcontent.com (151.205.17.57:443 / 151.205.17.57:443, host: google.cdn.steampipe.steamcontent.com): AuthenticateDepotID (251570) - Success!
==> configstore_log.txt <==
[2021-11-27 21:39:01] Flushed store 'userlocal' to '/mnt/steam/steam/userdata/REDACTED/config/localconfig.vdf'
==> content_log.txt <==
[2021-11-27 21:39:02] AppID 251570 update changed : Running,
[2021-11-27 21:39:02] AppID 251570 update changed : Running,Committing,
[2021-11-27 21:39:02] AppID 251570 starting commit from "/mnt/steam/steam/steamapps/shadercache/251570/swarm/downloads/" to "/mnt/steam/steam/steamapps/shadercache/251570/swarm/" : 1 updated, 0 moved, 0 deleted files
[2021-11-27 21:39:05] AppID 251570 update changed : Running,
[2021-11-27 21:39:05] AppID 251570 update changed : None
[2021-11-27 21:39:05] AppID 251570 state changed : Update Required,Update Queued,Update Started,
==> shader_log.txt <==
[2021-11-27 21:39:05] Committed bucket 228015 (AppID 251570) from 0 to 1075016100639355783.
==> content_log.txt <==
[2021-11-27 21:39:05] AppID 251570 scheduler finished : staying in schedule (result No Error, state 0x40a)
==> shader_log.txt <==
[2021-11-27 21:39:05] [AppID 251570] Finished Shader Swarm download job: No Error
[2021-11-27 21:39:06] [AppID 251570] Starting Shader Swarm download job.
==> configstore_log.txt <==
[2021-11-27 21:39:06] Flushed store 'install' to '/mnt/steam/steam/config/config.vdf'
==> content_log.txt <==
[2021-11-27 21:39:06] AppID 251570 state changed : Update Required,Update Queued,Update Running,Update Started,
[2021-11-27 21:39:06] AppID 251570 update changed : Running,
[2021-11-27 21:39:06] AppID 251570 update changed : Running,Reconfiguring,
[2021-11-27 21:39:08] HTTP (CDN,35) - google.cdn.steampipe.steamcontent.com (151.205.17.57:443 / 151.205.17.57:443, host: google.cdn.steampipe.steamcontent.com): qn-00201-sltnpastd03-0110104a-1.steam-content-dnld-1.qwilted-cds.cqloud.com/google.cdn.steampipe.steamcontent.com/depot/251570/manifest/5655415328003394686/5/3666157260602701859 - received 200 (OK) HTTP response
[2021-11-27 21:39:08] AppID 251570 update changed : Running,
[2021-11-27 21:39:08] AppID 251570 update changed : Running,Preallocating,
[2021-11-27 21:39:08] AppID 251570 update changed : Running,
==> shader_log.txt <==
[2021-11-27 21:39:06] [AppID 251570] Target manifest 5655415328003394686 of size 14432087 key SteamSwarm / G7:VulkanPipelinesV6.
[2021-11-27 21:39:06] Processing downloaded cache for AppID 251570, BucketID 228015.
[2021-11-27 21:39:08] Opened bucket file /mnt/steam/steam/steamapps/shadercache/251570/swarm//steam_shader_cache/bucket_228015.swarc, original cache type 2.
[2021-11-27 21:39:08] Writing 3136 hit entries to cache file /mnt/steam/steam/userdata/REDACTED/config/shaderhitcache/7e2ff036a05b1d69/4e72b67faf2c55a81064f0f669542d15/251570.
[2021-11-27 21:39:09] Writing depot of 3136 shaders back to original cache type 2, into target directory /mnt/steam/steam/steamapps/shadercache/251570.
[2021-11-27 21:39:09] Writing NVIDIA cache of 1 buckets into /mnt/steam/steam/steamapps/shadercache/251570.
==> content_log.txt <==
[2021-11-27 21:39:10] AppID 251570 update started : download 0/11544336, store 0/0, reuse 0/0, delta 0/0, stage 0/14432087
[2021-11-27 21:39:10] AppID 251570 update changed : Running,Downloading,Staging,
[2021-11-27 21:39:10] Downloading 14 chunks from depot 251570
==> shader_log.txt <==
[2021-11-27 21:39:10] CProcessShaderCacheJob done for AppID 251570.
==> content_log.txt <==
[2021-11-27 21:39:11] AppID 251570 update changed : Running,
[2021-11-27 21:39:11] AppID 251570 update changed : Running,Committing,
[2021-11-27 21:39:11] AppID 251570 starting commit from "/mnt/steam/steam/steamapps/shadercache/251570/swarm/downloads/" to "/mnt/steam/steam/steamapps/shadercache/251570/swarm/" : 1 updated, 0 moved, 0 deleted files
[2021-11-27 21:39:11] AppID 251570 update changed : Running,
[2021-11-27 21:39:11] AppID 251570 update changed : None
[2021-11-27 21:39:11] AppID 251570 state changed : Update Required,Update Queued,Update Started,
==> shader_log.txt <==
[2021-11-27 21:39:11] Committed bucket 1541162 (AppID 251570) from 0 to 5655415328003394686.
==> content_log.txt <==
[2021-11-27 21:39:11] AppID 251570 scheduler finished : staying in schedule (result No Error, state 0x40a)
==> shader_log.txt <==
[2021-11-27 21:39:11] [AppID 251570] Finished Shader Swarm download job: No Error
==> content_log.txt <==
[2021-11-27 21:39:12] AppID 251570 state changed : Update Required,Update Queued,Update Running,Update Started,
==> configstore_log.txt <==
[2021-11-27 21:39:12] Flushed store 'install' to '/mnt/steam/steam/config/config.vdf'
==> content_log.txt <==
[2021-11-27 21:39:12] AppID 251570 update changed : Running,
[2021-11-27 21:39:12] AppID 251570 update changed : Running,Reconfiguring,
[2021-11-27 21:39:16] AppID 251570 update changed : Running,
==> shader_log.txt <==
[2021-11-27 21:39:12] Processing downloaded cache for AppID 251570, BucketID 1541162.
[2021-11-27 21:39:16] Opened bucket file /mnt/steam/steam/steamapps/shadercache/251570/swarm//steam_shader_cache/bucket_1541162.swarc, original cache type 7.
[2021-11-27 21:39:16] Writing 19844 hit entries to cache file /mnt/steam/steam/userdata/REDACTED/config/shaderhitcache/SteamSwarm/VulkanPipelinesV6/251570.
[2021-11-27 21:39:17] Writing depot of 19844 shaders back to original cache type 7, into target directory /mnt/steam/steam/steamapps/shadercache/251570.
[2021-11-27 21:39:18] Writing Fossilize cache of 1 buckets into /mnt/steam/steam/steamapps/shadercache/251570/fozpipelinesv6.
==> content_log.txt <==
[2021-11-27 21:39:18] AppID 251570 update started : download 101980016/5094853104, store 0/0, reuse 0/0, delta 0/0, stage 238301845/11373702223
[2021-11-27 21:39:18] AppID 251570 update changed : Running,Downloading,Staging,
[2021-11-27 21:39:18] Downloading 15726 chunks from depot 251578
==> shader_log.txt <==
[2021-11-27 21:39:19] CProcessShaderCacheJob done for AppID 251570.
==> configstore_log.txt <==
[2021-11-27 21:39:20] Flushed store 'install' to '/mnt/steam/steam/config/config.vdf'
[2021-11-27 21:39:23] Flushed store 'install' to '/mnt/steam/steam/config/config.vdf'
[2021-11-27 21:39:53] Flushed store 'install' to '/mnt/steam/steam/config/config.vdf'
==> content_log.txt <==
[2021-11-27 21:39:57] Current download rate: 911.827 Mbps
[2021-11-27 21:40:11] AppID 251570 update changed : Running,
[2021-11-27 21:40:11] AppID 251570 update changed : Running,Verifying,
[2021-11-27 21:40:11] Update auto validation: enabled for all clients
[2021-11-27 21:40:11] Update auto validation (251578): clean bytes tally of 19277576006 bytes is > threshold of 2048 MB, not validating
[2021-11-27 21:40:11] AppID 251570 update changed : Running,
[2021-11-27 21:40:11] AppID 251570 update changed : Running,Committing,
[2021-11-27 21:40:11] AppID 251570 starting commit from "/mnt/steam/steam/steamapps/downloading/251570" to "/mnt/steam/steam/steamapps/common/7 Days To Die" : 6248 updated, 0 moved, 0 deleted files
[2021-11-27 21:40:14] AppID 251570 state changed : Fully Installed,Update Queued,Update Running,
[2021-11-27 21:40:14] AppID 251570 finished update, 1 mounted depots (BuildID 7108523) : 251578 (1562221519351495492),
[2021-11-27 21:40:14] AppID 251570 update changed : Running,
[2021-11-27 21:40:14] AppID 251570 update changed : None
[2021-11-27 21:40:14] AppID 251570 state changed : Fully Installed,Update Queued,
[2021-11-27 21:40:14] AppID 251570 no password cached for private beta "latest_experimental", falling back to public
[2021-11-27 21:40:14] AppID 251570 state changed : Fully Installed,
[2021-11-27 21:40:14] AppID 251570 scheduler finished : removed from schedule (result No Error, state 0xc)
==> cloud_log.txt <==
[2021-11-27 21:40:18] [AppID 251570] Starting sync (init,)
[2021-11-27 21:40:18] [AppID 251570] AutoCloud checking local state for user REDACTED
[2021-11-27 21:40:18] [AppID 251570] Not running AutoCloud. No config data found
[2021-11-27 21:40:18] [U:1:REDACTED] [AppID 251570] Init cache request complete
==> configstore_log.txt <==
[2021-11-27 21:40:23] Flushed store 'install' to '/mnt/steam/steam/config/config.vdf'
==> content_log.txt <==
[2021-11-27 21:40:58] Current download rate: 0.000 Mbps
==> cef_log.txt <==
[1127/214109.311539:ERROR:shared_image_representation.cc(151)] Attempt to read from an uninitialized SharedImage
==> controller_ui.txt <==
[2021-11-27 21:41:08] [252490]Non-Steam Controller Configs Enabled: 1
==> stats_log.txt <==
[2021-11-27 21:41:08] [AppID 252490] Loaded schema from disk.
[2021-11-27 21:41:08] [AppID 252490] Loading stats from disk...
==> steamwebhelper.log <==
[1127/214109.311539:ERROR:shared_image_representation.cc(151)] Attempt to read from an uninitialized SharedImage
==> configstore_log.txt <==
[2021-11-27 21:41:09] Flushed store 'userlocal' to '/mnt/steam/steam/userdata/REDACTED/config/localconfig.vdf'
==> stats_log.txt <==
[2021-11-27 21:41:09] [AppID 252490] Loading stats from disk...success
[2021-11-27 21:41:09] [AppID 252490] CAPIJobRequestUserStats - no stats data in server response, we must be up to date
==> content_log.txt <==
[2021-11-27 21:41:10] HTTP (SteamCache,15) - cache1-atl1.steamcontent.com (151.205.17.57:80 / 151.205.17.57:80, host: cache1-atl1.steamcontent.com): Closing connection
[2021-11-27 21:41:10] HTTP (CDN,35) - google.cdn.steampipe.steamcontent.com (151.205.17.57:443 / 151.205.17.57:443, host: google.cdn.steampipe.steamcontent.com): Closing connection
[2021-11-27 21:41:10] HTTP (SteamCache,1) - cache1-sea1.steamcontent.com (151.205.17.57:443 / 151.205.17.57:443, host: cache1-sea1.steamcontent.com): Closing connection
[2021-11-27 21:41:10] HTTP (SteamCache,276) - cache3-lax1.steamcontent.com (151.205.17.57:80 / 151.205.17.57:80, host: cache3-lax1.steamcontent.com): Closing connection
==> appinfo_log.txt <==
[2021-11-27 21:41:26] RequestAppInfoUpdate: AppIDs 252490
[2021-11-27 21:41:26] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:41:26] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
[2021-11-27 21:41:31] RequestAppInfoUpdate: AppIDs 252490
[2021-11-27 21:41:31] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:41:31] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
[2021-11-27 21:41:37] RequestAppInfoUpdate: AppIDs 252490
[2021-11-27 21:41:37] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:41:37] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
[2021-11-27 21:41:40] RequestAppInfoUpdate: AppIDs 252490
[2021-11-27 21:41:40] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:41:41] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
[2021-11-27 21:41:41] RequestAppInfoUpdate: AppIDs 252490
[2021-11-27 21:41:41] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:41:42] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
[2021-11-27 21:41:42] RequestAppInfoUpdate: AppIDs 252490
[2021-11-27 21:41:42] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:41:42] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
[2021-11-27 21:41:43] RequestAppInfoUpdate: AppIDs 252490
[2021-11-27 21:41:43] Requesting 1 apps, 0 packages (meta data, 0 prev attempts)
[2021-11-27 21:41:43] UpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)
Well, I finally bought a new 2tb SSD and created a new symlink to it from ~/.steam. Once I signed in I quit steam and rsynced all of steam/steamapps to copy my old game files over, then restarted.
With a new disk and a clean history, I have had no trouble installing new games again for the past week. Nothing in the SMART data or fsck indicated anything wrong with the old drive (1tb spinning metal drive), but I'll consider this solved.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
Trying to install a new game, I am getting a tiny 80x140 window that contains little more than the cancel button, and fails to actually install anything.

I have no problem updating exsting games in my library, or Proton, and have nearly 100gb free on /mnt/steam, which is the only drive specified as my library location (and is symlinked to ~/.steam)
So far I've tried:
That last attempt actually put the entire Steam window into an unresizable 80x140 box and forced me to quit it via the gnome menu and restore config.
Similar to https://steamcommunity.com/app/221410/discussions/0/3033726413411757742/ I'm getting spam in the error log about being unable to write .steam_exec_test.sh to directories that shouldn't be written to anyway; I have no idea why it's trying to write to all these places. Unlike Dragon, I do not have a wrong directory in my installation path.
The latest client updates from October 7 and 14 have "Fix checking for execute privileges when picking a Steam Library folder location" in the release notes, but all I see in the logs is fruitless spam.
After providing all this to Steam Support, they suggested I open an issue on github.
Steps for reproducing this issue:
.steam_exec_test.shto multiple directories appears in /mnt/steam/error.logUpdatesJob: finished OK, apps updated 0 (0 KB), packages updated 0 (0 KB)appears in /mnt/steam/steam/logs/appinfo_log.txt