Fix edge case around composer handling gendered facepalm emoji (#8686)

* Write tests around composer badly handling gendered facepalm emoji

* Commit export for tests to be happy

* Fix edge case around composer handling gendered facepalm emoji

* Fix offset calculations and make code more readable

test/editor/parts-test.ts

@@ -0,0 +1,35 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { EmojiPart, PlainPart } from "../../src/editor/parts";
+
+describe("editor/parts", () => {
+    describe("appendUntilRejected", () => {
+        const femaleFacepalmEmoji = "🤦‍♀️";
+
+        it("should not accept emoji strings into type=plain", () => {
+            const part = new PlainPart();
+            expect(part.appendUntilRejected(femaleFacepalmEmoji, "")).toEqual(femaleFacepalmEmoji);
+            expect(part.text).toEqual("");
+        });
+
+        it("should accept emoji strings into type=emoji", () => {
+            const part = new EmojiPart();
+            expect(part.appendUntilRejected(femaleFacepalmEmoji, "")).toBeUndefined();
+            expect(part.text).toEqual(femaleFacepalmEmoji);
+        });
+    });
+});