I have observed this with a lot of JavaScript countdown timers, in several browsers and always wondered what was happening behind the screen.
Select timer text.

And, it gets deselected on time change.

But, double click and select the timer text,

Magic! it won’t get deselected on time change.

So, what’s really happening?
In the first case, you are selecting the text node, which changes. In the second case, you are selecting the encompassing element, which is not changing.
For example, if you have a 0:12, when the time updates, the 0:12 might be 0:11.
So if you had selected that text, what you selected is then gone. But if you had selected the
it hasn’t changed to say, a span, so its still there and can remain selected.