First cut of pulling out the relevant parts of my original game to make a little framework.

assets/shaders/ui_shape_shader.frag

@@ -0,0 +1,12 @@
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+uniform float u_duration;
+uniform float u_time;
+uniform float u_time_end;
+
+void main() {
+  float tick = (u_time_end - u_time) / u_duration;
+  float blink = smoothstep(1.0, 0.5, tick);
+  vec4 color = vec4(blink, blink, blink, 1.0);
+  gl_FragColor = gl_Color * color;
+}