I opened a PDF with Pix, which displayed my art on a transparent layer. Fine. I make a new layer intending to fill with White. Grabbed the Fill too, made sure White was selected and clicked. Once.
For the next minute or so, I watched as Pixelmator filled that layer with white, then unfilled, then filled, then unfilled...filled and unfilled, possibly a dozen or more times.
Seems to have finally settled itself down. Any clue why this peculiar behavior?
Pix 3.5.1 on OS X 10.11.6 on an '09 Mac Pro with 16GB of RAM