Your system information

• Steam client version (build number or date): Client buildID 1778003620 (Steam Helper); streaming_client binary mtime 2026-05-05; streaming_log Client version 1773426488 (last logged session 2026-04-21)
• Distribution (e.g. Ubuntu): macOS 26.3.1 (Tahoe), build 25D2128, Apple Silicon arm64 — filing here per repo convention for macOS Steam client bugs
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes (macOS 26.3.1 is current)
• GPU: Apple M1 (integrated)

Please describe your issue in as much detail as possible:

The Steam Remote Play streaming_client on macOS no longer forwards mouse-wheel vertical scroll to the host. Mice with a horizontal thumb wheel reveal that horizontal scroll is being forwarded as vertical, indicating the wrong scroll axis is being read from Quartz events.

Expected: Rotating the mouse wheel up/down on the macOS client scrolls vertically in the streamed window on the host (this used to work).

Actual:

• Vertical mouse-wheel rotation → host receives nothing.
• Horizontal scroll wheel (MX Master 2S thumb wheel) → host receives vertical scroll.
• TouchBar / trackpad two-finger continuous scroll → host receives correct vertical scroll.
• Mouse movement, all mouse buttons (incl. wheel click), keyboard input → all forwarded correctly.
• Scroll works correctly in the main Steam app (Library, Store), in macOS desktop, and in every other macOS application — only streaming_client window is affected.

Hypothesis: streaming_client reads kCGScrollWheelEventDeltaAxis2 (horizontal) and treats it as vertical, while ignoring Axis1 (vertical) entirely. TouchBar/trackpad continuous scroll uses a different field (kCGScrollWheelEventPointDeltaAxis*) and remains functional, which is why TouchBar still works while mouse-wheel rotation does not.

Tested mice (both BLE, Logitech):

• Logitech G603 (vertical wheel only) → no scroll forwarded at all (consistent — never produces an Axis2 event)
• Logitech MX Master 2S (vertical + horizontal thumb wheel) → vertical lost, horizontal forwarded as vertical (consistent — produces Axis2 event for thumb wheel only)

Already ruled out:

• Steam Input (per-game and global) — disabled, no change
• Per-game and Big Picture controller settings — reset, no change
• macOS Reduce Motion / Stage Manager / Game Mode — all disabled
• Logi Options+ user-space — killed; Logi Options HID Driver Extension disabled in System Settings → Driver Extensions and confirmed [activated disabled] after restart
• Corsair iCUE5 user-space — killed
• TCC permissions — confirmed streaming_client has kTCCServiceListenEvent granted (auth_value=2, result=true in log show)
• Two physically different mice — same behaviour, so not hardware
• BT polling / HID — scroll works in every other Mac app from the same mice
• Window focus / fullscreen mode — clicks and movement work in streaming_client, only wheel rotation fails

Reddit thread reporting the same symptoms: https://www.reddit.com/r/Steam/comments/1t5500j/screwed_up_mouse_wheel_scrolling_in_steam_remote/

Steps for reproducing this issue:

1. On macOS 26.3.1 client, pair a Bluetooth mouse with a vertical scroll wheel (e.g. Logitech G603 or MX Master 2S). Verify scroll works in the macOS desktop and in the Steam Library — it does.
2. Start Steam Remote Play / Steam Link to a Windows host. Enter any game or the host desktop view.
3. Rotate the vertical scroll wheel → host shows no scroll.
4. (If mouse has a horizontal thumb wheel) push the thumb wheel left/right → host scrolls vertically.
5. Two-finger scroll on TouchBar/trackpad → host scrolls vertically correctly. This isolates the bug to mouse-wheel rotation events specifically.