Why is the focus visible when a lightDom element in a web component is clicked?

[Christophe Garcia]

I've created a simple web component that lets you create a custom button.

When clicked, I want the page title to be focused.

With button tag and a simple text content inside, the title is not visually focused. Same with my web component (in slot). That's cool.

With button tag and a tag span inside, the title is not visually focused. But, the title is visually focused with my web component (in slot). That's not cool.

I don't understand why the title is visually focused (which I don't want it to be), when I click on the lightDom element (e.g. the span). This doesn't happen when I click on my component directly (the shadowDOM part).

I expect my title to be focus but not visible-focus, without hiding by forcing.

I tried to look at this thread too: Why is focus-visible applied on page load

// Template web component
const template = document.createElement('template');
template.innerHTML = `
      <style>
        button {
          background-color: lightgreen;
          padding: 10px;
        }
      </style>
      <button>
        <slot></slot>
      </button>`;

// Class web component
class MyBtn extends HTMLElement {
  constructor() {
    super();

    this.attachShadow({
      mode: 'open'
    });

    this.shadowRoot.appendChild(template.content.cloneNode(true));
  }
}

// web component definition
window.customElements.define('my-btn', MyBtn);

// Set focus to title on click
function focusTitle() {
  const h1 = document.getElementById('title');
  h1.setAttribute('tabindex', -1);
  h1.focus();
}

// Add click event listener
document.getElementById('btn1').addEventListener('click', focusTitle);
document.getElementById('btn2').addEventListener('click', focusTitle);

<h1 id="title">Test redirection focus</h1>
<button id="btn1" style="padding: 10px">
    <span>(button > span) Click me and I don't visually focus title</span>
    (button > simple text content) Click me and I don't visually focus title
</button>

<my-btn id="btn2">
  <span>(MyBtn > span) Click me and I DO visually focus title</span>
  (MyBtn > simple text content) Click me and I don't visually focus title
</my-btn>