Thank you for your post and video.
In the video, I noticed there is an interrupt occurring every half second.
Do you have any plug-ins installed? If so, temporarily disable them and try again.
Do you have a OnFirstWindow script trigger active? If so, what does the script trigger do? If you turn off the script trigger, does the issue still occur?
Any other information you can provide about your computing environment may be helpful in narrowing down the possible causes.
No plugins, but you are right there is an ontimer script running which I removed and it seems to work. However the ontimer script with the refresh should not impact Layout mode windows. Even if it is running on a another window in the background. Seems like weird behaviour!
If FileMaker Pro is being interrupted at a set interval, it will definitely have an effect on other windows, including the active window, as it executes the script and then returns to the active window.