Here's a newly-introduced addition to Photoshop's roster of buggy behaviors: Masking is now intermittently broken when using the zoom and pan functions to perform up-close edits to images.
Once it happens, this bug makes it completely impossible to see even a vague suggestion of the current mask shape for affected portions of the image, forcing the user to apply their mask changes partway through tweaking so they can see the mask outline again to continue editing.
Why do I say this is a newly-introduced bug? Because my workflow hasn't changed in many months, but this has only started happening to me some time in the past couple of weeks and now happens quite regularly (ie. multiple times per day if I'm spending at least a few hours per day editing). I never saw this behavior at all under beta; it seems only to have been introduced with the final release.
The issue can be replicated as follows:
* Open an image with a subject that's too tricky to mask entirely automatically (in other words, any image that doesn't have a night-and-day difference between subject and background luminance and color). In my case, that's silver, grey or chrome wheel rims shot against a white background, where Photoshop's selection algorithms can't tell the difference between light gray and near-white even though the human eye easily can. I'm not sure if the image resolution matters, but I'm not editing especially high-res images; out of camera they're ~18 megapixels, and typically I'm editing at more like ~3500x3500px or so.)
* Click the "Select and mask" button to enter masking mode and enable the Overlay mask with high-quality preview and 100% opacity. (I doubt the color makes a difference, but I use 255,0,0 red.)
* Click "Select subject" to have Photoshop attempt to select and mask the subject automatically.
* Zoom in on an area of the image which has been masked incorrectly by Photoshop and start to correct the outline using the polygonal lasso tool. (I'm not sure if the exact zoom level matters but my editing is usually between 200% and 500% zoom levels.)
* Pan the image by dragging the mouse pointer to an edge of the screen without closing your selection, continuing to click on the image occasionally to keep redrawing the selection as you do so.
* At some point -- again, this problem while relatively frequent is intermittent -- you will find that when you pan the mask will not be shown correctly for a random area of the image, but rather both masked and unmasked areas will both be shown in red, making it impossible to distinguish between the two any more.
* There will be a straight dividing line between the area of the mask that's shown correctly and that which is shown incorrectly, and panning away from that area then returning to it will not fix the issue. The *only* way to get the mask to show correctly again once this happens is to close and apply your adjustment to the mask without completing tracing of your chosen outline first (since you can no longer see where to trace.) Once the path is closed, the mask will instantly show correctly again.
* I've only seen this happen with the division between correct and incorrect mask display being horizontal (ie. during vertical panning of the image), but it's possible that it may happen on the other axis as well.
I don't yet have a screenshot of the issue but will endeavor to get and add one to this thread the next time this happens to me.