Fix fluid simulation not starting and invisible rendering

Two bugs: (1) useFluidSim's effect ran before the <pre> ref was available
because the mounted gate rendered a placeholder without the ref. Split into
FluidSimInner child component that only mounts after the DOM is ready.
(2) ASCII renderer thresholds were too high - density 0.01-0.2 produced
colored spaces (invisible). Use explicit thresholds aligned with the color
gradient and bump ambient source intensity for visible activity on load.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/FluidSimulation/ascii-renderer.ts

@@ -9,7 +9,7 @@ const BLOCKS = [" ", "\u2591", "\u2592", "\u2593", "\u2588"];
 
 // Color palette: density maps through cyan -> green -> yellow -> white
 function densityToColor(d: number): string {
-  if (d < 0.01) return "";
+  if (d < 0.005) return "";
 
   const clamped = Math.min(d, 1);
 
@@ -38,9 +38,11 @@ function densityToColor(d: number): string {
 }
 
 function densityToBlock(d: number): string {
-  if (d < 0.01) return " ";
-  const idx = Math.min(Math.floor(d * BLOCKS.length), BLOCKS.length - 1);
-  return BLOCKS[idx];
+  if (d < 0.005) return " ";
+  if (d < 0.03) return BLOCKS[1]; // ░
+  if (d < 0.1) return BLOCKS[2]; // ▒
+  if (d < 0.3) return BLOCKS[3]; // ▓
+  return BLOCKS[4]; // █
 }
 
 export interface RenderTarget {

src/components/FluidSimulation/index.tsx

@@ -8,19 +8,12 @@ const DESKTOP_GRID = { w: 80, h: 40 };
 const MOBILE_GRID = { w: 40, h: 25 };
 const MOBILE_BREAKPOINT = 768;
 
-export default function FluidSimulation() {
+/**
+ * Inner component that only mounts after the DOM is ready.
+ * This ensures useFluidSim's effect runs when preRef.current exists.
+ */
+function FluidSimInner({ isMobile }: { isMobile: boolean }) {
   const preRef = useRef<HTMLPreElement>(null);
-  const [isMobile, setIsMobile] = useState(false);
-  const [mounted, setMounted] = useState(false);
-
-  useEffect(() => {
-    setMounted(true);
-    const check = () => setIsMobile(window.innerWidth < MOBILE_BREAKPOINT);
-    check();
-    window.addEventListener("resize", check);
-    return () => window.removeEventListener("resize", check);
-  }, []);
-
   const grid = isMobile ? MOBILE_GRID : DESKTOP_GRID;
 
   const { toggleFaucet, clear } = useFluidSim(preRef, {
@@ -33,30 +26,6 @@ export default function FluidSimulation() {
     viscosity: 0.000001,
   });
 
-  if (!mounted) {
-    return (
-      <div className="w-full border border-border bg-background">
-        <pre
-          className="text-xs leading-none select-none overflow-hidden p-0 m-0"
-          style={{
-            fontSize: isMobile ? "10px" : "11px",
-            lineHeight: "1",
-          }}
-          aria-hidden="true"
-        >
-          {Array(grid.h)
-            .fill(null)
-            .map((_, j) => (
-              <span key={j}>
-                {" ".repeat(grid.w)}
-                {j < grid.h - 1 ? "\n" : ""}
-              </span>
-            ))}
-        </pre>
-      </div>
-    );
-  }
-
   return (
     <div className="w-full">
       {/* The simulation canvas */}
@@ -108,3 +77,44 @@ export default function FluidSimulation() {
     </div>
   );
 }
+
+export default function FluidSimulation() {
+  const [isMobile, setIsMobile] = useState(false);
+  const [mounted, setMounted] = useState(false);
+
+  useEffect(() => {
+    setMounted(true);
+    const check = () => setIsMobile(window.innerWidth < MOBILE_BREAKPOINT);
+    check();
+    window.addEventListener("resize", check);
+    return () => window.removeEventListener("resize", check);
+  }, []);
+
+  const grid = isMobile ? MOBILE_GRID : DESKTOP_GRID;
+
+  if (!mounted) {
+    return (
+      <div className="w-full border border-border bg-background">
+        <pre
+          className="text-xs leading-none select-none overflow-hidden p-0 m-0"
+          style={{
+            fontSize: "11px",
+            lineHeight: "1",
+          }}
+          aria-hidden="true"
+        >
+          {Array(grid.h)
+            .fill(null)
+            .map((_, j) => (
+              <span key={j}>
+                {" ".repeat(grid.w)}
+                {j < grid.h - 1 ? "\n" : ""}
+              </span>
+            ))}
+        </pre>
+      </div>
+    );
+  }
+
+  return <FluidSimInner isMobile={isMobile} />;
+}

src/components/FluidSimulation/use-fluid-sim.ts

@@ -76,14 +76,14 @@ export function useFluidSim(
     const time = Date.now() * 0.001;
     const ax = Math.floor(gridWidth * 0.2);
     const ay = Math.floor(gridHeight * 0.8);
-    addSource(state, ax, ay, 0.05, 2);
+    addSource(state, ax, ay, 0.2, 3);
     addVelocity(
       state,
       ax,
       ay,
-      Math.sin(time) * 0.5,
-      -0.5 + Math.cos(time * 0.7) * 0.3,
-      2
+      Math.sin(time) * 1.0,
+      -1.0 + Math.cos(time * 0.7) * 0.5,
+      3
     );
 
     stepFluid(state, dt, diffusion, viscosity, iterations);