So, you set Idle Timeout to 0
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And UI3 is still eventually redirecting you to timeout.htm?
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The only reason this should happen is if your UI3 settings were lost (in which case you'd open UI Settings and find that Idle Timeout was no longer set to 0).
Settings are stored in the browser's Local Storage which means the settings are unique to a particular browser on a particular device. They are not synchronized across multiple devices. Additionally if you ever disable or clear your cookies, this usually includes Local Storage as well, so you'd lose UI3 settings at that point.