UserAgent: Mozilla/5.0 (X11; Linux x86_64; rv:62.0) Gecko/20100101 Firefox/62.0

Steps to reproduce the problem:
* Add the attached JS file in a blank HTML page or go to http://jsfiddle.net/s7x8yu0e/.  The script run requestIdleCallback every 500ms, and logs a message when the idle callback is called.

* Open the console to verify that a message is logged every 500ms.

* Focus the page with the mouse (= click the jsfiddle page frame), and move the mouse around.  Stop moving the mouse.  Repeat.

What is the expected behavior?
The message should be logged every 500ms (more or less).  The "count" variable should stay at 0.

What went wrong?
Sometimes, when the mouse stops moving and pointer becomes still, the callback is not called until the pointer moves again.  We can see the setInterval callback was still correctly called every 500ms because the "count" variable is incremented.

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 69.0.3497.100  Channel: stable
OS Version: ArchLinux
Flash Version: 

This happens quite often on Linux.  A colleague tried on macOS (laptop + trackpad, no mouse) and reproduced the issue almost everytime.