[IMP] mockContentHelper: use focus to change window.mockContentHelper

This avoid having to rely on the obscure selectRange method.

Odoo task 2882521

Part-of: #3456

src/components/composer/composer/composer.ts

@@ -422,6 +422,7 @@ export class Composer extends Component<Props, SpreadsheetChildEnv> {
     this.shouldProcessInputEvents = false;
 
     if (this.props.focus !== "inactive") {
+      this.contentHelper.el.focus();
       this.contentHelper.selectRange(0, 0); // move the cursor inside the composer at 0 0.
     }
     const content = this.getContent();

src/components/composer/content_editable_helper.ts

@@ -1,9 +1,10 @@
 import { HtmlContent } from "./composer/composer";
 
 export class ContentEditableHelper {
+  // todo make el private and expose dedicated methods
   el: HTMLElement;
-  constructor(el: HTMLElement | null) {
-    this.el = el!;
+  constructor(el: HTMLElement) {
+    this.el = el;
   }
 
   updateEl(el: HTMLElement) {

tests/components/__mocks__/content_editable_helper.ts

@@ -27,10 +27,6 @@ export class ContentEditableHelper {
     this.colors = {};
   }
   selectRange(start: number, end: number) {
-    // TODO: find a way not to depend on selectRange to gain focus and push mockContentHelper
-    this.el!.focus();
-    // @ts-ignore
-    window.mockContentHelper = this;
     this.manualRange = true;
     // We cannot set the cursor position beyond the text of the editor
     if (!this.el || !this.el.textContent || start < 0 || end > this.el.textContent.length) return;
@@ -100,6 +96,8 @@ export class ContentEditableHelper {
   private attachEventHandlers() {
     if (this.el === null) return;
     this.el.addEventListener("keydown", (ev: KeyboardEvent) => this.onKeyDown(this.el!, ev));
+    // @ts-ignore
+    this.el.addEventListener("focus", (ev: FocusEvent) => (window.mockContentHelper = this));
   }
 
   /**