From 045358c02759d2212692a3fac17f8ee01c28625a Mon Sep 17 00:00:00 2001
From: Arvind Satyanarayan <arvindsatya1@gmail.com>
Date: Fri, 29 Apr 2022 14:54:52 -0400
Subject: [PATCH] fix: prevent selection compilers from side effecting
 selection defs

---
 src/compile/selection/parse.ts | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/compile/selection/parse.ts b/src/compile/selection/parse.ts
index aeccb98a48..d816af1505 100644
--- a/src/compile/selection/parse.ts
+++ b/src/compile/selection/parse.ts
@@ -52,9 +52,10 @@ export function parseUnitSelection(model: UnitModel, selDefs: SelectionParameter
       events: isString(defaults.on) ? parseSelector(defaults.on, 'scope') : array(duplicate(defaults.on))
     } as any);
 
+    const def_ = duplicate(def); // defensive copy to prevent compilers from causing side effects
     for (const c of selectionCompilers) {
       if (c.defined(selCmpt) && c.parse) {
-        c.parse(model, selCmpt, def);
+        c.parse(model, selCmpt, def_);
       }
     }
   }