In a document set to 16 bit float colorspace, filling the canvas with white via backspace or similar produces unexpected results Steps to reproduce: 1. Paint several strokes on canvas with "Addition" color mode, until you have color values over "1". 2. Fill canvas with white. 3. Use color picker to sample an area that previously had superwhite values. You'd expect the values to be "1", but it seems the fill operation has worked in "Addition" mode, so you have superwhite values. This seems rather unexpected.
This isn't a bug. The same thing happens with all other painting tools. This is so you can apply a gradient with a blending mode or the like.