Linux: Firefox exposes reserved Xbox Series buttons and swaps X/Y

Environment

• Host OS: Linux
• Virtual controller provider: libvirtualhid through Sunshine
• Profile: Xbox Series
• Browser: Firefox
• Reported identity: 045e-0b13-Sunshine (libvirtualhid) X-Box Series Controller
• Tester: https://app.lizardbyte.dev/gamepad-tester/

Observed behavior

• Firefox exposes 20 buttons.
• B17 is the Share button and is now labeled correctly by gamepad-helper.
• B18 and B19 remain visible but do not represent real Xbox Series controls.
• The physical X and Y buttons are swapped.
• Chromium exposes only B17 beyond the standard buttons and reports X/Y correctly.

Source-level explanation

The Linux uinput backend advertises KEY_RECORD for Share and also advertises reserved BTN_C and BTN_Z capabilities to preserve sparse Xbox button slots. Firefox starts with 17 canonical buttons and then appends every advertised nonstandard key capability, producing:

• B17: KEY_RECORD
• B18: reserved BTN_C
• B19: reserved BTN_Z

Firefox only applies its Xbox-specific BTN_NORTH/BTN_WEST correction when an ancestor driver is named xpad or microsoft. A uinput-created virtual controller does not have either driver ancestry despite using Microsoft VID/PID.

Firefox source:
https://searchfox.org/firefox-main/source/dom/gamepad/linux/LinuxGamepad.cpp

Expected behavior

• Firefox exposes the standard 17 buttons plus Share at B17.
• No phantom B18/B19 controls are presented.
• X and Y match the physical Xbox layout.

Constraints

Steam currently handles this virtual Xbox Series controller correctly and is the priority consumer. Do not remove reserved capabilities or swap emitted X/Y without validating Steam and Chromium. Keep any compatibility change Linux-specific and avoid changing the Windows backend.