fix: sync #dpr immediately after controller reinit on alphaEnabled toggle

package.json

@@ -22,6 +22,7 @@
   "scripts": {
     "build": "vite build",
     "dev": "vite build --watch",
+    "dev:demo": "bun run --cwd ./demo dev",
     "test": "vitest",
     "test:run": "vitest run",
     "test:coverage": "vitest run --coverage",

src/core/simulation.ts

@@ -131,6 +131,7 @@ export class FluidSimulation {
 
   #config: FluidConfig;
   #mouse: MouseState = { x: 0, y: 0, dx: 0, dy: 0, targetX: 0, targetY: 0, moved: false };
+  #mouseSeeded = false;
 
   #source: Source | null = null;
 
@@ -219,6 +220,13 @@ export class FluidSimulation {
   }
 
   handleMove(x: number, y: number, strength = 1): void {
+    if (!this.#mouseSeeded) {
+      // Seed position on first call so the initial delta isn't measured from (0,0).
+      this.#mouse.x = this.#mouse.targetX = x;
+      this.#mouse.y = this.#mouse.targetY = y;
+      this.#mouseSeeded = true;
+      return;
+    }
     this.#mouse.moved = true;
     this.#mouse.dx = (x - this.#mouse.targetX) * strength;
     this.#mouse.dy = (y - this.#mouse.targetY) * strength;

src/fluid-controller.ts

@@ -134,7 +134,7 @@ export class FluidController {
     if (this.#worker) {
       this.#worker.postMessage({ type: 'resize', width, height, dpr: effectiveDpr });
     } else {
-      this.#sim!.resize(width, height, effectiveDpr);
+      this.#sim?.resize(width, height, effectiveDpr);
     }
   }
 

src/react/useFluid.ts

@@ -80,6 +80,14 @@ export function useFluid(
     });
     controllerRef.current = controller;
 
+    // Immediately sync dimensions so #dpr is correct from the first frame.
+    // The ResizeObserver only fires when the container *size* changes, so it
+    // won't re-fire on an alphaEnabled/webGPUEnabled toggle where the container
+    // hasn't moved. Without this call the simulation keeps #dpr=1 and all
+    // pointer-to-UV coordinate transforms are wrong until something else
+    // triggers a resize.
+    if (initW > 0 && initH > 0) controller.resize(initW, initH);
+
     // Forward container resizes to the simulation — reads clampedDprRef so pixelRatio changes are picked up
     const ro = new ResizeObserver((entries) => {
       for (const entry of entries) {