Room and user mentions for plain text editor (#10665)

* update useSuggestion

* update useSuggestion-tests

* add processMention tests

* add test

* add getMentionOrCommand tests

* change mock href for codeQL reasons

* fix TS issue in test

* add a big old cypress test

* fix lint error

* update comments

* reorganise functions in order of importance

* rename functions and variables

* add endOffset to return object

* fix failing tests

* update function names and comments

* update comment, remove delay

* update comments and early return

* nest mappedSuggestion inside Suggestion state and update test

* rename suggestion => suggestionData

* update comment

* add argument to findSuggestionInText

* make findSuggestionInText return mappedSuggestion

* fix TS error

* update comments and index check from === -1 to < 0

* tidy logic in increment functions

* rename variable

* Big refactor to address multiple comments, improve behaviour and add tests

* improve comments

* tidy up comment

* extend comment

* combine similar returns

* update comment

* remove single use variable

* fix comments

cypress/e2e/composer/composer.spec.ts

@@ -15,9 +15,11 @@ limitations under the License.
 */
 
 /// <reference types="cypress" />
+import { EventType } from "matrix-js-sdk/src/@types/event";
 
 import { HomeserverInstance } from "../../plugins/utils/homeserver";
 import { SettingLevel } from "../../../src/settings/SettingLevel";
+import { MatrixClient } from "../../global";
 
 describe("Composer", () => {
     let homeserver: HomeserverInstance;
@@ -181,6 +183,81 @@ describe("Composer", () => {
             });
         });
 
+        describe("Mentions", () => {
+            // TODO add tests for rich text mode
+
+            describe("Plain text mode", () => {
+                it("autocomplete behaviour tests", () => {
+                    // Setup a private room so we have another user to mention
+                    const otherUserName = "Bob";
+                    let bobClient: MatrixClient;
+                    cy.getBot(homeserver, {
+                        displayName: otherUserName,
+                    }).then((bob) => {
+                        bobClient = bob;
+                    });
+                    // create DM with bob
+                    cy.getClient().then(async (cli) => {
+                        const bobRoom = await cli.createRoom({ is_direct: true });
+                        await cli.invite(bobRoom.room_id, bobClient.getUserId());
+                        await cli.setAccountData("m.direct" as EventType, {
+                            [bobClient.getUserId()]: [bobRoom.room_id],
+                        });
+                    });
+
+                    cy.viewRoomByName("Bob");
+
+                    // Select plain text mode after composer is ready
+                    cy.get("div[contenteditable=true]").should("exist");
+                    cy.findByRole("button", { name: "Hide formatting" }).click();
+
+                    // Typing a single @ does not display the autocomplete menu and contents
+                    cy.findByRole("textbox").type("@");
+                    cy.findByTestId("autocomplete-wrapper").should("be.empty");
+
+                    // Entering the first letter of the other user's name opens the autocomplete...
+                    cy.findByRole("textbox").type(otherUserName.slice(0, 1));
+                    cy.findByTestId("autocomplete-wrapper")
+                        .should("not.be.empty")
+                        .within(() => {
+                            // ...with the other user name visible, and clicking that username...
+                            cy.findByText(otherUserName).should("exist").click();
+                        });
+                    // ...inserts the username into the composer
+                    cy.findByRole("textbox").within(() => {
+                        // TODO update this test when the mentions are inserted as pills, instead
+                        // of as text
+                        cy.findByText(otherUserName, { exact: false }).should("exist");
+                    });
+
+                    // Send the message to clear the composer
+                    cy.findByRole("button", { name: "Send message" }).click();
+
+                    // Typing an @, then other user's name, then trailing space closes the autocomplete
+                    cy.findByRole("textbox").type(`@${otherUserName} `);
+                    cy.findByTestId("autocomplete-wrapper").should("be.empty");
+
+                    // Send the message to clear the composer
+                    cy.findByRole("button", { name: "Send message" }).click();
+
+                    // Moving the cursor back to an "incomplete" mention opens the autocomplete
+                    cy.findByRole("textbox").type(`initial text @${otherUserName.slice(0, 1)} abc`);
+                    cy.findByTestId("autocomplete-wrapper").should("be.empty");
+                    // Move the cursor left by 4 to put it to: `@B| abc`, check autocomplete displays
+                    cy.findByRole("textbox").type(`${"{leftArrow}".repeat(4)}`);
+                    cy.findByTestId("autocomplete-wrapper").should("not.be.empty");
+
+                    // Selecting the autocomplete option using Enter inserts it into the composer
+                    cy.findByRole("textbox").type(`{Enter}`);
+                    cy.findByRole("textbox").within(() => {
+                        // TODO update this test when the mentions are inserted as pills, instead
+                        // of as text
+                        cy.findByText(otherUserName, { exact: false }).should("exist");
+                    });
+                });
+            });
+        });
+
         it("sends a message when you click send or press Enter", () => {
             // Type a message
             cy.get("div[contenteditable=true]").type("my message 0");