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F08 — DPI / runtime rescale ​

Logical vs physical coordinates, and what happens when the scale changes under a live window. The four platforms give four different answers — including X11's "no runtime answer at all", which is a deliverable here, not a gap.

Requirements ​

  1. Continuously display (render into the frame) and log: logical size, physical (pixel) size, and the current scale factor; crisp 1-physical-px hairlines at the window edge make scaling artifacts visible.
  2. Handle a runtime scale change and log the full event sequence:
    • Wayland: implement BOTH paths and switch on advertised globals: wp_fractional_scale_v1 (preferred-scale in 1/120ths) + wp_viewport for buffer-size decoupling, AND the integer wl_surface.set_buffer_scale fallback driven by wl_surface.preferred_buffer_scale/wl_output.scale. Log which path is active.
    • Win32: Per-Monitor-v2 awareness (manifest or SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2)); on WM_DPICHANGED, honor the suggested rect (lParam) and log old/new DPI + rect.
    • macOS: observe backingScaleFactor via viewDidChangeBackingProperties/NSWindowDidChangeBackingPropertiesNotification; log convertRectToBacking: round-trips.
    • X11: demonstrate that there is no runtime mechanism: read Xft.dpi from resources and RANDR physical dimensions at startup, change the system scale, show the running window receives nothing. That finding IS the deliverable.
  3. Survive a monitor-to-monitor drag between different scales without blurriness or wrong-size buffers (Tier C where it needs real monitors; headless weston can be configured with two outputs at different scales — do that for Wayland).

Instrumentation ​

scale_changed scale=… path=…, dpi_changed old=… new=… suggested=WxH, plus resize size=WxH scale=S everywhere sizes change.

Findings to record ​

  • The native coordinate unit per platform and where the scale is first learnable (the "created at wrong scale then rescaled" problem — log whether it happens).
  • The full event order for a scale change (feeds event-sequences.md).
  • Fractional-scale buffer-size math on Wayland (rounding rules) and what viewport decouples.
  • The X11 absence, documented with the startup-snapshot workaround frameworks use.

Verification ​

Wayland: Tier A (headless weston with two outputs, scales 1 and 1.5/2). X11: Tier A (the absence-proof). Win32: A[wine] for the WM_DPICHANGED plumbing (Wine emulates per-monitor DPI imperfectly — label it), real mixed-DPI drag Tier C. macOS: scale observation A[ssh], monitor drag Tier C.