diff --git a/resources/shaders/common.glsl b/resources/shaders/common.glsl
index 17f454f..e7f4e03 100644
--- a/resources/shaders/common.glsl
+++ b/resources/shaders/common.glsl
@@ -121,8 +121,8 @@ void setOutputColor(vec4 outColor) { cogl_color_out = outColor; }
 // The Shell.GLSLEffect uses straight alpha blending. This helper method allows
 // compositing color values in the shader in the same way.
 vec4 alphaOver(vec4 under, vec4 over) {
-  float alpha = over.a + under.a * (1.0 - over.a);
-  return vec4((over.rgb * over.a + under.rgb * under.a * (1.0 - over.a)) / alpha, alpha);
+  float alpha = mix(under.a, 1.0, over.a);
+  return vec4(mix(under.rgb * under.a, over.rgb, over.a) / alpha, alpha);
 }
 
 // ------------------------------------------------------------------------- color helpers