Mask feathering varies with centre view zoom and pan
The effect of mask feathering seems to vary depending on which parts of the image are currently visible. See attached screenshots.
1. Select an image with a bright sky and a bright patch approaching the horizon from below.
2. Zoom out to 100%
3. Select the tone curve module
4. Select "drawn mask" from "blend"
5. Apply a gradient in the drawn mask panel to select the sky.
6. Enable mask preview (shows affected area in yellow on greyscale image)
7. Adjust feathering radius, mask opacity and mask contrast to extend the masked area from the sky into the brighter foreground areas that approach the horizon.
8. Switch off mask preview.
9. Adjust the tone curve to map all pixels to zero brightness using a completely level "curve". This is just to demonstrate the problem.
10. Observe which parts of the image have been darkened by the extreme tone curve.
11. Zoom to 1:1 on a foreground area that the feathering algorithm has "bled" the mask into. Observe masked area changes compared to 100% view.
12. Pan the center view around and notice how the masked areas change.
The problem is also evident when the mask preview control is switched on, but demonstrating it using the steps above shows that it's not a bug in the mask preview.
Note: This is with version 2.6.2, but there's no 2.6.2 in the dropdown for "affected version" on redmine so I've attributed it to git master.