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F13 — CSD & decoration modes (Wayland only) ​

On X11, Win32, and macOS the system draws window decorations; on Wayland the client does, unless the compositor volunteers server-side decorations. This is the single largest "the platform made it the app's problem" divergence, and it is Wayland-only by design: the other three platforms record "N/A — SSD" plus one line on their customization hooks (WM_NCCALCSIZE/DwmExtendFrameIntoClientArea; NSWindowStyleMask/titlebarAppearsTransparent; Motif hints/_GTK_FRAME_EXTENTS).

Requirements ​

  1. Negotiate zxdg_decoration_manager_v1: request server-side, log the compositor's configure mode=… answer; honor mode switches at runtime.
  2. When SSD is denied (or the protocol is absent — log which), draw a minimal CSD: a title bar (solid rect + window title) that starts an interactive move via xdg_toplevel.move on pointer-down, and 8 edge/corner zones that start xdg_toplevel.resize with the right edge enum; close button sends nothing — it requests client shutdown (ties into F14's vetoable close).
  3. Account for the geometry consequences: xdg_surface.set_window_geometry so the compositor knows frame vs content; shadows/margins may be omitted but note what real CSD adds (subsurfaces or margin drawing, input region beyond the visible frame).
  4. Build a second variant linking libdecor (libdecor_decorate) for comparison — LOC, behavior under the same compositors, what it handles that the minimal CSD doesn't (themes, shadows, double-click-to-maximize, touch).

NOTE

libdecor is an allowed exception to the no-helper-libraries rule for exactly this variant — measuring what the helper buys is the point.

Instrumentation ​

decoration mode=ssd|csd source=protocol|absent, csd_hit zone=…, move_start, resize_start edge=….

Findings to record ​

  • Decoration-mode answers per compositor (headless weston, then mutter / kwin / sway via the manual queue — mutter famously refuses SSD).
  • LOC: minimal CSD vs libdecor variant vs "free" SSD elsewhere.
  • What minimal CSD cannot do (snap layouts, theme consistency, a11y) — the real cost ledger.
  • The set_window_geometry contract and what breaks without it.

Verification ​

Tier A under headless weston (protocol-level negotiation + programmatic move/resize won't complete without a real pointer — capture the requests in the WAYLAND_DEBUG trace). Interactive drag/resize behavior and per-compositor findings: Tier C (GNOME, KDE, sway sessions).