Steam Controller 2026: trackpad widgets in Button HUD (Phase 3b)

[petegordon]

What

Add two trackpad widgets to the Button HUD showing left + right trackpad state in real-time: finger position (X, Y), touch active flag, contact area / pressure.

Why

The Steam Controller has two trackpads as primary inputs — they're the main reason people pick the controller. The visualizer body has hasTouchpad: false in its profile and even if true would only show one touchpad like DualSense's single one. A side-by-side widget pair in the HUD (similar style to the existing stick dots) would surface them in both the in-overlay HUD and the popout window.

The driver already parses both trackpads — parsed.touchpad is a 2-element array [{active, id, x, y, area}, ...]. So this is purely visualizer + HUD work, no driver changes needed.

Acceptance criteria

• HUD has two trackpad widgets (L and R) showing finger position as a dot inside a square frame
• Touch state visually indicated (e.g. dot only visible when active: true, or dot brightness scaled by area)
• Updates at the same rate as buttons/sticks
• Forwarded to the popout HUD window via the existing button-hud-state IPC channel
• Profile-conditional: only shown for controllers whose profile declares trackpads (so DualSense / Xbox / Switch Pro don't get empty widgets)

Refs

• Steam Controller 2026: identify + parse via WebHID without Steam Input #8 (origin Steam Controller issue)
• Steam Controller: switch IMU to raw gyro/accel — all 3 axes working #12 (latest IMU work — driver already returns trackpad data)
• apps/overlay/src/js/app.js → updateButtonHud
• apps/overlay/src/index.html (Button HUD markup)
• apps/overlay/src/js/button-hud-window.js (popout HUD renderer)