Bug reproduced in V10.4.1 and persisting through V11.0.1
A support case with the identification [CASE:3713364] was created
[...] I was able to reproduce the issue, and consequently, I filed a report with our development team raising the issues [...]
Probably closely related to Toggling DynamicUpdating aborts dynamics with UpdateInterval yet different enough to deserve a separate question.
Please follow those steps to reproduce the problem:
evaluate
(CreateDocument[#]; #) & @ Slider @ Dynamic @ xconfirm that both sliders are in synch
disable dynamics in one of the notebooks, e.g. via Evaluation menu
move the slider in the second notebook
(!) move mouse over the disabled one again (it glitches to
0value but is still disabled)enable dynamic updating
(!) try to move that slider, changes are reflected in the second one but the appearance/state of this one is not updated
