From 91dcdabad9b86b6ed26c8f213c52a9c25df1c707 Mon Sep 17 00:00:00 2001
From: Rob Eisenberg <EisenbergEffect@users.noreply.github.com>
Date: Tue, 1 Mar 2022 14:39:15 -0500
Subject: [PATCH] feat: handle existing shadow roots when upgrading (#5679)

* feat: handle existing shadow roots when upgrading

* Change files

Co-authored-by: EisenbergEffect <roeisenb@microsoft.com>
---
 ...-19ee7846-01d0-4dc8-babc-29a4537526a5.json |  7 +++++
 .../src/components/controller.spec.ts         | 27 ++++++++++++++++++-
 .../fast-element/src/components/controller.ts | 17 +++++++++---
 3 files changed, 46 insertions(+), 5 deletions(-)
 create mode 100644 change/@microsoft-fast-element-19ee7846-01d0-4dc8-babc-29a4537526a5.json

diff --git a/change/@microsoft-fast-element-19ee7846-01d0-4dc8-babc-29a4537526a5.json b/change/@microsoft-fast-element-19ee7846-01d0-4dc8-babc-29a4537526a5.json
new file mode 100644
index 00000000000..adaa1c19fae
--- /dev/null
+++ b/change/@microsoft-fast-element-19ee7846-01d0-4dc8-babc-29a4537526a5.json
@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "feat: handle existing shadow roots when upgrading",
+  "packageName": "@microsoft/fast-element",
+  "email": "roeisenb@microsoft.com",
+  "dependentChangeType": "patch"
+}
diff --git a/packages/web-components/fast-element/src/components/controller.spec.ts b/packages/web-components/fast-element/src/components/controller.spec.ts
index 172d741baee..33bcd7a35da 100644
--- a/packages/web-components/fast-element/src/components/controller.spec.ts
+++ b/packages/web-components/fast-element/src/components/controller.spec.ts
@@ -388,6 +388,7 @@ describe("The Controller", () => {
 
         expect(element.shadowRoot?.contains(style)).to.equal(false);
     });
+
     it("should attach and detach the HTMLStyleElement supplied to .addStyles() and .removeStyles() to the shadowRoot", () => {
         const { controller, element } = createController({
             shadowOptions: {
@@ -510,5 +511,29 @@ describe("The Controller", () => {
             controller.removeBehaviors([behavior], true);
             expect(behavior.bound).to.equal(false);
         });
-    })
+    });
+
+    context("with pre-existing shadow dom on the host", () => {
+        it("re-renders the view during connect", async () => {
+            const name = uniqueElementName();
+            const element = document.createElement(name);
+            const root = element.attachShadow({ mode: 'open' });
+            root.innerHTML = 'Test 1';
+
+            document.body.append(element);
+
+            new FASTElementDefinition(
+                class TestElement extends FASTElement {
+                    static definition = {
+                        name,
+                        template: html`Test 2`
+                    };
+                }
+            ).define();
+
+            expect(root.innerHTML).to.equal("<!---->Test 2");
+
+            document.body.removeChild(element);
+        });
+    });
 });
diff --git a/packages/web-components/fast-element/src/components/controller.ts b/packages/web-components/fast-element/src/components/controller.ts
index 6730134b29f..521d2457a9a 100644
--- a/packages/web-components/fast-element/src/components/controller.ts
+++ b/packages/web-components/fast-element/src/components/controller.ts
@@ -26,6 +26,7 @@ export class Controller extends PropertyChangeNotifier {
     private boundObservables: Record<string, any> | null = null;
     private behaviors: Map<Behavior<HTMLElement>, number> | null = null;
     private needsInitialization: boolean = true;
+    private hasExistingShadowRoot = false;
     private _template: ElementViewTemplate | null = null;
     private _styles: ElementStyles | null = null;
     private _isConnected: boolean = false;
@@ -159,10 +160,16 @@ export class Controller extends PropertyChangeNotifier {
         const shadowOptions = definition.shadowOptions;
 
         if (shadowOptions !== void 0) {
-            const shadowRoot = element.attachShadow(shadowOptions);
+            let shadowRoot = element.shadowRoot;
 
-            if (shadowOptions.mode === "closed") {
-                shadowRoots.set(element, shadowRoot);
+            if (shadowRoot) {
+                this.hasExistingShadowRoot = true;
+            } else {
+                shadowRoot = element.attachShadow(shadowOptions);
+
+                if (shadowOptions.mode === "closed") {
+                    shadowRoots.set(element, shadowRoot);
+                }
             }
         }
 
@@ -434,7 +441,9 @@ export class Controller extends PropertyChangeNotifier {
             // If there's already a view, we need to unbind and remove through dispose.
             this.view.dispose();
             (this as Mutable<this>).view = null;
-        } else if (!this.needsInitialization) {
+        } else if (!this.needsInitialization || this.hasExistingShadowRoot) {
+            this.hasExistingShadowRoot = false;
+
             // If there was previous custom rendering, we need to clear out the host.
             for (let child = host.firstChild; child !== null; child = host.firstChild) {
                 host.removeChild(child);