From ff0b93a7fb4ca416629f51cc233078494aaf5f60 Mon Sep 17 00:00:00 2001
From: Nicholas Rice <3213292+nicholasrice@users.noreply.github.com>
Date: Wed, 19 Jul 2023 16:12:00 -0700
Subject: [PATCH] fix: prevent array splice notification on non-mutating
 splice() calls (#6786)

* always project splices

* Change files

* Update packages/web-components/fast-element/src/observation/arrays.spec.ts

---------

Co-authored-by: nicholasrice <nicholasrice@users.noreply.github.com>
---
 ...-44cfe95a-1f60-4e2e-a3f1-1d02aba63bab.json |  7 +++++++
 .../src/observation/arrays.spec.ts            | 19 +++++++++++++++++++
 .../fast-element/src/observation/arrays.ts    |  2 +-
 3 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 change/@microsoft-fast-element-44cfe95a-1f60-4e2e-a3f1-1d02aba63bab.json

diff --git a/change/@microsoft-fast-element-44cfe95a-1f60-4e2e-a3f1-1d02aba63bab.json b/change/@microsoft-fast-element-44cfe95a-1f60-4e2e-a3f1-1d02aba63bab.json
new file mode 100644
index 00000000000..86d2fd7d98a
--- /dev/null
+++ b/change/@microsoft-fast-element-44cfe95a-1f60-4e2e-a3f1-1d02aba63bab.json
@@ -0,0 +1,7 @@
+{
+    "type": "prerelease",
+    "comment": "Prevent notification of array splices when operation does not mutate array values",
+    "packageName": "@microsoft/fast-element",
+    "email": "nicholasrice@users.noreply.github.com",
+    "dependentChangeType": "prerelease"
+}
diff --git a/packages/web-components/fast-element/src/observation/arrays.spec.ts b/packages/web-components/fast-element/src/observation/arrays.spec.ts
index b43b84b3d12..27da19775f0 100644
--- a/packages/web-components/fast-element/src/observation/arrays.spec.ts
+++ b/packages/web-components/fast-element/src/observation/arrays.spec.ts
@@ -433,6 +433,25 @@ describe("The ArrayObserver", () => {
 
         expect(wasCalled).to.be.false;
     })
+
+    it("should not deliver splices for .splice() when .splice() does not change the items in the array", async () => {
+        ArrayObserver.enable();
+        const array = [1,2,3,4,5];
+        const observer = Observable.getNotifier(array);
+        let splices;
+
+        observer.subscribe({
+            handleChange(source, args) {
+                splices = args
+            }
+        });
+
+        array.splice(0, array.length, ...array);
+
+        await Updates.next();
+
+        expect(splices.length).to.equal(0);
+    })
 });
 
 describe("The array length observer", () => {
diff --git a/packages/web-components/fast-element/src/observation/arrays.ts b/packages/web-components/fast-element/src/observation/arrays.ts
index 7388aa1724d..f6c2226f948 100644
--- a/packages/web-components/fast-element/src/observation/arrays.ts
+++ b/packages/web-components/fast-element/src/observation/arrays.ts
@@ -611,7 +611,7 @@ let defaultSpliceStrategy: SpliceStrategy = Object.freeze({
             if (changes === void 0) {
                 return emptyArray;
             }
-            return changes.length > 1 ? project(current, changes) : changes;
+            return project(current, changes);
         }
 
         return resetSplices;