Selecting mask whilst viewing alpha changes image aspect.

Thought so. I normally have it set to fit and my PM window is normally slightly wider than 16x9 so its obvious there is a change. If you have the PM adjusted to almost exactly 16x9 (excluding the tabs and buttons areas) then there is not an obvious difference when you select the controls in the EC.

In my case, I am doing lots of green screen shots and I'm using the opacity masks to garbage mask parts of the set. With my green screens shots on their own layer, and Ultrakey applied, I can view the alpha and scrub through the shots looking out for tiny bits of white which indicate mics or lighting stands which are just creeping into the shots and I might have missed with my masks. Unfortunately, when viewing the alpha, I don't have access to the mask controls in the PM.

Tried lots of different codecs and sequence settings - can't get it to do it on 2017 but it happens on both 2018 versions.

Ann, if you set your program monitor zoom to 10%, view the alpha, add an opacity mask and then click between the 'Opacity' field and the 'Mask (1)' field...

...does the program monitor change at all?

W10

PP2018.1.1 and PP2018.1.2

Both have same issue. I created a new project in 2017 and it was fine. Since you are not seeing it, I'll do a bit more investigation and try to pin down the cause.

Nope cannot reproduce on W10. Guessing this is 2018.1.2?

Even used a gopro clip.