diff --git a/core/toolbox/toolbox.ts b/core/toolbox/toolbox.ts
index 31bb2b6363d..4979fdfa40f 100644
--- a/core/toolbox/toolbox.ts
+++ b/core/toolbox/toolbox.ts
@@ -1172,6 +1172,7 @@ Css.register(`
 
 /* Category tree in Toolbox. */
 .blocklyToolbox {
+  box-sizing: border-box;
   user-select: none;
   -ms-user-select: none;
   -webkit-user-select: none;