I use a webcam with my Octoprint and it normally works great. However trying to get it to go full screen and NOT magnified is a problem. When I mouseover the webcam image it magnifies so if I double click it to go full screen the image is magnified which I want regular, not magnified.
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Unless I am misunderstanding your description, this seems to be a function of the Webcam extras Plugin you have installed. I would say that that is how it works really. If you are already zoomed in and you double click its going to be zoomed in. If you are not zoomed in, it will not be.
I think you could also be seeing the results from a slow double click setup with your mouse. If you are not zoomed in and one were to do a sorta lazy double click, I can make it go into the zoomed state even though I was not when I decided to double click.
Might also be that your double click is actually doing 3 clicks. This is my bet.
I also sorta would expect it to lose the zoom if I move my mouse outside the video window but I can also see why that might be annoying if you are just moving the mouse near the edge and it suddenly zooms out. I think it's one of those design decisions that maybe should have forced the dev to create a user preferences in setup but sometimes, just keeping it simple is better.
As I am sure you noted, an additional click will zoom you out. Seems pretty reasonable to work that way. Although I can understand where you might want it to read your mind and know that you really want 3 clicks and not 2.
Not the answer you were hoping for... You could also open an issue on the plugin's git issues page.