ID 151259 - Prevent auto zoom not followed when using the show overscan option

Problem summary

When enabling in the Viewer Settings 'Prevent auto zoom' option, then viewing an image and enabling the 'show overscan option', this will still change the zoom level within the Viewer based upon the image size, altering the viewport position in the Viewer.

Customer reported version
nuke.11.3v4

Customer reported platform
n_a_linux

Steps to reproduce

1. Open the attached script.
2. View the RotoPaint node in the Viewer
3. Set the Viewer zoom level to around 5% and play the timeline.
4. View the results in the Viewer.
5. Right-click on the Viewer, confirm that Prevent Auto Zoom is active, and activate 'Show Overscan'
6. Play the timeline again
Result: The Viewer will zoom in and out, based upon the image size and not the Viewer size.

Workaround
Unknown. 

Reproduced by support

Reproduced by support on:

Expected behaviour
The zoom level should only be modified when "Prevent Auto Zoom" is checked.

Actual behaviour
The Viewer zoom level changes if both 'Prevent Auto Zoom" AND "show overscan' are enabled at the same time.