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@ -237,21 +237,23 @@ There's also `window.onscroll` event.
## Focus/blur on a window
Theoretically, there are `window.focus()` and `window.blur()` methods to focus/unfocus on a window. Also there are `focus/blur` events that allow to focus a window and catch the moment when the visitor switches elsewhere.
Theoretically, there are `window.focus()` and `window.blur()` methods to focus/unfocus on a window. And there are also `focus/blur` events that allow to catch the moment when the visitor focuses on a window and switches elsewhere.
In the past evil pages abused those. For instance, look at this code:
Although, in practice they are severely limited, because in the past evil pages abused them.
For instance, look at this code:
```js run
window.onblur = () => window.focus();
```
When a user attempts to switch out of the window (`blur`), it brings it back to focus. The intention is to "lock" the user within the `window`.
When a user attempts to switch out of the window (`window.onblur`), it brings the window back into focus. The intention is to "lock" the user within the `window`.
So, there are limitations that forbid the code like that. There are many limitations to protect the user from ads and evils pages. They depend on the browser.
So browsers had to introduce many limitations to forbid the code like that and protect the user from ads and evils pages. They depend on the browser.
For instance, a mobile browser usually ignores that call completely. Also focusing doesn't work when a popup opens in a separate tab rather than a new window.
For instance, a mobile browser usually ignores `window.focus()` completely. Also focusing doesn't work when a popup opens in a separate tab rather than a new window.
Still, there are some things that can be done.
Still, there are some use cases when such calls do work and can be useful.
For instance: