diff --git a/src/index.ts b/src/index.ts
index bfd3121..a4c3666 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1322,7 +1322,8 @@ export class Context {
     ) {
       value = low + ((this.mousePos.x - base.x) * (high - low)) / base.w;
       if (step !== null && step !== 0) {
-        value = ((value + step / 2) / step) * step;
+        value = Math.round((value - low) / step) * step + low;
+        value = Math.max(low, Math.min(value, high));
       }
     }
     // clamp and store value, update res