🐛 Restart progressive render on canvas background change (#11643)

Cancel Partial/ViewportReady before invalidating tiles so a mid-flight
pass does not leave background-only holes. Closes #11642.

frontend/src/app/render_wasm/api.cljs

@@ -2211,6 +2211,10 @@
   [background]
   (when (initialized?)
     (let [rgba (sr-clr/hex->u32argb background 1)]
+      ;; Background is baked into every tile. Cancel Partial/ViewportReady so we
+      ;; do not continue a progressive pass whose tile cache was just cleared —
+      ;; that drops already-finished tiles from the queue and leaves bg-only holes.
+      (stop-progressive-render!)
       (h/call wasm/internal-module "_set_canvas_background" rgba)
       (request-render "set-canvas-background"))))