Linux: Browser Gamepad API omits DualShock 4 and DualSense touchpad button

Environment

• Host OS: Linux
• Virtual controller provider: libvirtualhid through Sunshine
• Affected profiles: DualShock 4 and DualSense
• Consumers: Firefox and Chromium
• Tester: https://app.lizardbyte.dev/gamepad-tester/

Observed behavior

Neither browser exposes the touchpad click button for the virtual DualShock 4 or DualSense.

Example DualSense identities:

• Chromium: Sunshine (libvirtualhid) PS5 Controller (STANDARD GAMEPAD Vendor: 054c Product: 0ce6)
• Firefox: 054c-0ce6-Sunshine (libvirtualhid) PS5 Controller

The gamepad-helper mapping already assigns PlayStation button index 17 to the touchpad, so the missing value occurs before presentation when the browser's Gamepad.buttons collection is populated.

Expected behavior

The touchpad click should be exposed as a browser gamepad button and rendered with the existing touchpad mapping.

Investigation boundary

Compare raw browser Gamepad API state with hidraw/evdev reports and each browser's standard-mapping behavior. Do not change the public controller mapping or Windows backend unless the captured report path proves that is required.