use fragment shader based dot grid background

shaders/dot_grid.frag

@@ -0,0 +1,32 @@
+#include <flutter/runtime_effect.glsl>
+precision mediump float;
+
+uniform vec2 uScreenOrigin;      // screenOffset in pixels
+uniform float uScaledSpacing;     // spacing * scale in pixels
+uniform vec2 uPhasePx;            // phase shift in pixels: mod(-canvasOffset * scale, scaledSpacing)
+uniform float uRadiusPx;          // dot radius in pixels: size * scale
+uniform vec4 uDotColor;           // RGBA 0..1
+uniform vec4 uBgColor;            // RGBA 0..1
+
+out vec4 fragColor;
+
+void main() {
+  vec2 p = FlutterFragCoord().xy - uScreenOrigin; // local pixel coords within the painted rect
+
+  // LOD: if dots become sub-pixel or spacing too dense, draw only background
+  if (uScaledSpacing < 1.0 || uRadiusPx < 0.25) {
+    fragColor = uBgColor;
+    return;
+  }
+
+  // Periodic grid in screen space with phase to account for canvasOffset
+  vec2 rem = mod(p + uPhasePx, uScaledSpacing);
+  vec2 d = min(rem, vec2(uScaledSpacing) - rem);
+  float dist = length(d);
+
+  // Analytic AA around the circle edge (~1px transition)
+  float edge = uRadiusPx;
+  float alpha = 1.0 - smoothstep(edge - 1.0, edge + 1.0, dist);
+
+  fragColor = mix(uBgColor, uDotColor, alpha);
+}