Take a website with some kind of notifications system showing the user a count of their unread notifications or similar. (Facebook, Google+, GMail et c.)
Now perform an action in one browser tab, that changes or clears the count. Does this immediately propagate to all other tabs you have open on the site?
If not – this example is for you!
As it happens, this is really easy to do in
All of the below properties or methods, when requested/called in JavaScript, will trigger the browser to synchronously calculate the style and layout*. This is also called reflow or layout thrashing, and is common performance bottleneck.
Generally, all APIs that synchronously provide layout metrics will trigger forced reflow / layout. Read on for additional cases and details.
elem.offsetLeft, elem.offsetTop, elem.offsetWidth, elem.offsetHeight, elem.offsetParent