Steam client hangs on macOS 26.3 (Tahoe) — main thread deadlocked in fgets() within steamui.dylib

steamclosed

ValveSoftware/steam-for-linux#12958 · opened 2026-03-06 by User7142 · updated 2026-03-06 · 1 comments · github

UUser7142 2026-03-06 github

Your system information

Steam client version: 1769025840 (also tested with Beta build 1772686978)
Distribution: macOS 26.3 (Build 25D125), Apple M1 Max (MacBookPro18,2)
Opted into Steam client beta?: Tested both Stable and Beta — same issue
Have you checked for system updates?: Yes, macOS 26.3 is current
Steam Logs: Pasted inline below (macOS log path: ~/Library/Application Support/Steam/logs/)
GPU: Apple M1 Max (integrated), ANGLE Metal Renderer

Please describe your issue in as much detail as possible:

Steam launches but never shows a window — no login dialog, no main window, nothing. The process runs in the background with full CPU subsystems initialized (CEF/Chromium 126, GPU process, renderer, network service), but the UI never appears.

Root cause identified via sample: The main thread of steam_osx is permanently blocked in a fgets() call deep inside steamui.dylib:

Thread_main (com.apple.main-thread):
  steam_osx + 0x13fc4
    steamui.dylib + 0x543cf8
      steamui.dylib + 0x5eee3c
        steamui.dylib + 0xa260ec
          steamui.dylib + 0xb01d34
            steamui.dylib + 0xa52160
              steamui.dylib + 0x968fc
                fgets (libsystem_c.dylib)
                  __read_nocancel (libsystem_kernel.dylib)

Because the main thread is deadlocked:

The internal WebSocket server on port 27060 accepts TCP connections but never responds (TLS handshake times out)
SteamUI JavaScript fails repeatedly with failed to reach open state after 8 retries, then gives up
No browser windows are created (Invalid browser dimensions: 0 x 0 in webhelper.txt)
The transport layer reports: SteamUI thread frame stalled for: 578752 ms and Connection rejected

Last working session: 2026-02-21 on the same macOS 26.3 installation (installed 2026-02-05).

What was tried (all unsuccessful):

Clean reinstall of Steam.AppBundle (fresh download)
Fresh Steam.app from steam.dmg (bootstrapper v4.0 -> v5.1)
Complete config reset (deleted config/, appcache/, registry.vdf)
Font cache reset (atsutil databases -removeUser)
Deleting htmlcache, CEF cache, shared memory files, steam.pipe
Steam Client Beta (build 1772686978)
Stopping Docker Desktop (ruled out as cause)

Steps for reproducing this issue:

Run macOS 26.3 (Tahoe, Build 25D125) on Apple Silicon (M1 Max)
Launch Steam via /Applications/Steam.app
Steam process starts, Dock icon appears, but no window is ever shown
Run sample <pid> — shows main thread permanently blocked in fgets() inside steamui.dylib

console_log.txt

[2026-03-06 10:25:16] Client version: 1769025840
[2026-03-06 10:25:16] Loaded SDL version release-3.4.0-98-gb1362f8a4
[2026-03-06 10:25:16] ThreadGetProcessExitCode: no such process 42846
[2026-03-06 10:25:16] ChildProcessQuery: failed with code -1
[2026-03-06 10:25:16] CGpuTopology: failed to run gpu topology query
[2026-03-06 10:25:16] Failed to init SteamVR because it isn't installed
[2026-03-06 10:25:24] Console Log Start

[2026-03-06 10:03:25] src/clientdll/steamengine.cpp (2844) : Assertion Failed: CSteamEngine::BMainLoop appears to have stalled > 15 seconds without event signalled
[2026-03-06 10:03:25] src/common/tcpconnection.cpp (993) : 0 == iRet
[2026-03-06 10:13:04] src/common/processpipe_posix.cpp (519) : Assertion Failed: CCrossProcessPipe::BWrite: 32 (Broken pipe)
[2026-03-06 10:13:04] src/steamUI/webuitransportcontroller.cpp (175) : Failed to connect to websocket

webhelper.txt (last session)

[2026-03-06 10:25:24] Client version: no bootstrapper found
[2026-03-06 10:25:24] Starting message loop
[2026-03-06 10:25:25] CreateBrowser 460054938 type:12 flags:0  (-2147483648, -2147483648) 0x0
[2026-03-06 10:25:25] UB-65536: AfterCreated handle:65536 type:12: (0, 0) 0x0
[2026-03-06 10:25:26] Invalid browser dimensions: 0 x 0
[2026-03-06 10:25:26] UB-65536: SetName: SP Shared JS Context

webhelper_js.txt (WebSocket connection failures)

[2026-03-06 10:25:28] SteamUI: INFO: Storing new config params ?LOCAL_HOSTNAME=MacBook_M1Max&LANGUAGE=german&COUNTRY=&LAUNCHER_TYPE=0&CLIENT_SESSION=1772786900&IN_CLIENT=true&USE_POPUPS=true&IN_STEAMUI_SHARED_CONTEXT=true&PLATFORM=macos
[2026-03-06 10:25:28] SteamUI: INFO: setting locale: de
[2026-03-06 10:25:47] SteamUI: WARNING: failed to reach open state
[2026-03-06 10:25:47] SteamUI: WARNING: connect attempt failed: 2 - failed to reach open state
[2026-03-06 10:25:47] SteamUI: INFO: connect retry: attempt:0/8 backoff:1000
[2026-03-06 10:25:47] SteamUI: WARNING: failed to reach open state
[2026-03-06 10:25:47] SteamUI: WARNING: connect attempt failed: 2 - failed to reach open state
[2026-03-06 10:25:47] SteamUI: INFO: connect retry: attempt:0/8 backoff:1000
(... retries up to attempt 7/8, then:)
[2026-03-06 10:26:xx] SteamUI: WARNING: websocket connect retry: limit exceeded, bailing - steamUI
[2026-03-06 10:26:xx] SteamUI: INFO: Connection status: disconnected

transport_steamui.txt

[2026-03-06 10:25:16] Client version: 1769025840
[2026-03-06 10:25:16] WebUITransportController: Initialized
[2026-03-06 10:25:26] WebUITransport: Websocket connection from: https://steamloopback.host
[2026-03-06 10:25:26] WebUITransport: Connection from: 127.0.0.1:52732

[2026-03-06 10:03:25] WebUITransport: TCP connection request
[2026-03-06 10:03:25] WebUITransport: Connection rejected
[2026-03-06 10:13:04] Warning: SteamUI thread frame stalled for: 578752 ms
[2026-03-06 10:13:04] ReportTransportFailure: count: 0

transport_client.txt

[2026-03-06 10:25:26] Client version: 1769025840
[2026-03-06 10:25:26] WebUITransport: Websocket connection from: https://steamloopback.host
[2026-03-06 10:25:26] WebUITransport: Connection from: 127.0.0.1:52733

[2026-03-06 10:03:25] WebUITransport: TCP connection request
[2026-03-06 10:03:25] WebUITransport: Connection rejected

sample output (main thread deadlock proof)

Analysis of sampling steam_osx (pid 39214) every 1 millisecond
Process:         steam_osx [39214]
Path:            /Users/USER/Library/Application Support/Steam/*/steam_osx
Load Address:    0x102ab0000
Identifier:      com.valvesoftware.steam
Version:         6.1
Code Type:       ARM64
Platform:        macOS
Parent Process:  launchd [1]

Date/Time:       2026-03-06 10:01:03.372 +0100
Launch Time:     2026-03-06 09:57:35.938 +0100
OS Version:      macOS 26.3 (25D125)
Report Version:  7
Analysis Tool:   /usr/bin/sample

Physical footprint:         38.1M
Physical footprint (peak):  38.5M

Call graph:
    870 Thread_15595095   DispatchQueue_1: com.apple.main-thread  (serial)
    + 870 start  (in dyld) + 7184
    +   870 ???  (in steam_osx)  + 0x13fc4
    +     870 ???  (in steam_osx)  + 0x1469c
    +       870 ???  (in steamui.dylib)  + 0x543cf8
    +         870 ???  (in steamui.dylib)  + 0x542a00
    +           870 ???  (in steamui.dylib)  + 0x5eee3c
    +             870 ???  (in steamui.dylib)  + 0xa260ec
    +               870 ???  (in steamui.dylib)  + 0xb01d34
    +                 870 ???  (in steamui.dylib)  + 0xb04b4c
    +                   870 ???  (in steamui.dylib)  + 0xa51fb4
    +                     870 ???  (in steamui.dylib)  + 0xa4f9e4
    +                       870 ???  (in steamui.dylib)  + 0xa50c54
    +                         870 ???  (in steamui.dylib)  + 0xa52160
    +                           870 ???  (in steamui.dylib)  + 0x85cc4
    +                             870 ???  (in steamui.dylib)  + 0x85bec
    +                               870 ???  (in steamui.dylib)  + 0x968fc
    +                                 870 fgets  (in libsystem_c.dylib) + 272
    +                                   870 __srefill1  (in libsystem_c.dylib) + 36
    +                                     870 _sread  (in libsystem_c.dylib) + 32
    +                                       870 __sread  (in libsystem_c.dylib) + 24
    +                                         870 __read_nocancel  (in libsystem_kernel.dylib) + 8

connection_log.txt (last session)

[2026-03-06 10:25:16] Client version: 1769025840
[2026-03-06 10:25:16] Log session started
[2026-03-06 10:25:16] [Logged Off, 0, 0] [U:1:0] CCMInterface::SetSteamID( [U:1:0] )
[2026-03-06 10:25:16] Connectivity test: Starting test, fetching 'http://test.steampowered.com/204'
[2026-03-06 10:25:16] Connectivity test: result=Connected
[2026-03-06 10:25:16] IPv6 HTTP connectivity test - TIMEOUT
[2026-03-06 10:25:16] IPv6 UDP connectivity test - FAILED, no addresses resolved

Note: Connectivity test passes but login is never attempted because the main thread is deadlocked.

bootstrap_log.txt (last session)

[2026-03-06 10:25:01] Startup - updater built Sep 25 2025 08:53:01
[2026-03-06 10:25:01] Startup - Steam Client launched with: '/Applications/Steam.app/Contents/MacOS/steam_osx'
[2026-03-06 10:25:02] Startup - updater built Jan 20 2026 08:10:52
[2026-03-06 10:25:02] Startup - Steam Client launched with: '/Users/thomas/Library/Application Support/Steam/Steam.AppBundle/Steam/Contents/MacOS/steam_osx'
[2026-03-06 10:25:13] Installation wird überprüft ...
[2026-03-06 10:25:13] Verifying file sizes only
[2026-03-06 10:25:13] Verification complete
[2026-03-06 10:25:13] Not updating bootstrapper: No update necessary: current version 5.1, package version 5.1

Kkisak-valve maintainer 2026-03-06 github

Hello @User7142, issues with the Steam for Mac client are out of scope for this issue tracker. Please use Steam Support to reach the larger Steam support team.

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