Hi, kalimarks!
Thanks for reaching out to Microsoft Community and posting your inquiry regarding the pause/play button on the F5 key for your Surface Laptop 3. We're sorry if you encountered troubles about its functionality. Thanks for taking the time to let us know about this and allowing us to help you out here.
We understand that the rest of the buttons will work like the backlit keys, volume adjustment, display brightness adjustment, etc. It is just the pause/play function. The key will work as long as there is media played on the window. Say for example, you're playing a video on the Youtube website, you can use the pause/play key to halt or continue playing the online video.
If this answers your question, please let our Community know by marking this post as an answer. Otherwise, don't hesitate to respond to this thread if there’s anything else that needs to be addressed.
Kind regards,
Janette_Z