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F09 — Output enumeration & hotplug ​

What a window can know about the displays it lives on, and whether it finds out when that changes at runtime.

Requirements ​

  1. At startup, enumerate all outputs and log per output: name/identifier, position, mode (resolution + refresh), scale, and physical size where available:
    • Wayland: wl_output (+ xdg_output for logical geometry where offered).
    • X11: RandR (XRRGetMonitors/XRRGetScreenResources).
    • Win32: EnumDisplayMonitors + GetMonitorInfo + GetDpiForMonitor.
    • macOS: NSScreen.screens (+ CGDisplay IDs).
  2. Log which output(s) the window currently occupies and track changes:
    • Wayland: wl_surface.enter/leave (the only platform that tells the surface directly).
    • X11/Win32/macOS: derive from window geometry vs monitor rects; log the derivation.
  3. Handle hotplug at runtime: an output appearing/disappearing while the window runs must be logged (output_added/output_removed) without crash or stale state — including the case where the window's current output vanishes.

Instrumentation ​

output id=… geom=… scale=… refresh=… per enumeration; output_added/output_removed; surface_output enter|leave id=….

Findings to record ​

  • The information model per platform (what's logical vs physical, what's missing — e.g. exact refresh on Wayland before wl_output.mode).
  • Hotplug event order, and what happens to a window whose output disappears.
  • Whether enumeration requires a window/connection or is global.

Verification ​

Wayland: Tier A — headless weston can be started with multiple outputs; output hot-add is scriptable with the headless backend's --output-count is static, so use weston + wlr-randr-equivalent where possible, else record hot-add as Tier C on a real session. X11: Tier A for enumeration under Xvfb (single output; note the limitation), RandR mode-change events scriptable via xrandr on Xephyr if available. Win32/macOS: enumeration A[wine]/A[ssh]; physical hotplug Tier C.