Place windows at map time so stream restarts never flash

Diagnosis: individual tiles flickered when their stream died and mpv
relaunched (confirmed via status: a PID dropping to 0 and coming back).
The restarted window mapped at the video's native size wherever sway
dropped it, and the corrective loop only snapped it into its cell on the
next tick — up to 2s later on the relaxed cadence.

Fix, at the source instead of racing the map:
- Pre-install a per-slot for_window rule (matched on each mpv's unique
  window title, anchored so slot-1 never matches slot-10) so sway
  positions and sizes the window synchronously the moment it maps.
- Pass --geometry=WxH per tile so mpv opens at the tile size rather
  than resizing itself to the video's native size on load.

The placeLoop remains as a corrective backstop for mid-life drift.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/compositor/compositor.go

@@ -185,3 +185,17 @@ func (c *Client) Place(ctx context.Context, pid int, r Rect) error {
 func (c *Client) PrepareForMPV(ctx context.Context) error {
 	return c.run(ctx, `for_window [app_id="mpv"] floating enable, border none`)
 }
+
+// PlaceOnMap installs a for_window rule that positions any window with the
+// given title at rect the moment it maps. This is what keeps a restarting
+// stream from flashing at the wrong place/size: sway applies the geometry
+// synchronously at map time, before the daemon's corrective loop ever sees
+// the window. Re-installing a rule for the same title replaces it. The title
+// is anchored (^...$) so slot-1 never matches slot-10.
+func (c *Client) PlaceOnMap(ctx context.Context, title string, r Rect) error {
+	cmd := fmt.Sprintf(
+		`for_window [title="^%s$"] floating enable, border none, move absolute position %d %d, resize set %d %d`,
+		title, r.X, r.Y, r.W, r.H,
+	)
+	return c.run(ctx, cmd)
+}

internal/daemon/daemon.go

@@ -177,6 +177,13 @@ func (d *Daemon) applyLayout(ctx context.Context, name string) error {
 
 	for _, s := range specs {
 		p := player.New(s.slot, s.name, s.url, cfg.Player, d.runDir, d.log)
+		p.TileW, p.TileH = s.rect.W, s.rect.H
+		// Pre-install the placement rule so sway positions this slot's window
+		// (including after mid-life mpv restarts) the moment it maps — no
+		// wrong-place flash while waiting for the corrective loop.
+		if err := d.comp.PlaceOnMap(ctx, p.Title(), s.rect); err != nil {
+			d.log.Warn("could not preinstall placement rule", "slot", s.slot, "err", err)
+		}
 		players = append(players, p)
 		places = append(places, placement{p: p, rect: s.rect})
 

internal/player/player.go

@@ -25,6 +25,10 @@ type Player struct {
 	Slot int    // grid cell index (row-major)
 	Name string // camera name, for logs/titles
 	URL  string // RTSP(S) source
+	// TileW/TileH, when set (before Supervise), are passed as --geometry so
+	// mpv opens its window at the tile size instead of the video's native
+	// size — a restarting stream then maps already-sized for its cell.
+	TileW, TileH int
 
 	cfg     config.Player
 	ipcPath string
@@ -94,6 +98,12 @@ func (p *Player) args() []string {
 		// the audio decoder saves CPU per stream on the Pi.
 		args = append(args, "--no-audio")
 	}
+	if p.TileW > 0 && p.TileH > 0 {
+		// Open the window at the tile size; --geometry overrides mpv's
+		// resize-to-video-size on load, so the compositor never has to snap
+		// the window back into its cell.
+		args = append(args, fmt.Sprintf("--geometry=%dx%d", p.TileW, p.TileH))
+	}
 	if p.cfg.MaxFPS > 0 {
 		args = append(args, fmt.Sprintf("--vf=fps=%d", p.cfg.MaxFPS))
 	}