I'm using Chrome and this happens on a number of WordPress sites.
When I'm adding an image to a post, the browser tab will freeze on the Insert Media screen after I've dragged an image to upload it.
I can then open a new tab and visit the WP dashboard and then edit the post where I left off, but the frozen tab remains frozen.
This happens on a number of WordPress sites, each with different plugins/themes.
Any idea what is causing this?