My thumbnails have turned black?

[Risk]

When I export a transparent png file the background of the thumbnail is black. When I open the image it's white. I've made 100's of images and in the past, they show the thumbnails with a white background. Only new images have changed to black. The older ones are still white. I check the file in gimp and it is indeed transparent. What has changed?>

[Ofnuts]

What color is the "transparent"? If there are transparent pixels in the PNG image, their color is whatever the image viewer decides (white, black, checkerrboard...).

The same should apply to the thumbnails, but the thumbnail creation could ignore the pixel opacity bits, in which case you would see the color set by Gimp in these transparent pixels. Depending on your choice for the Save color values for transparent pixels option, you see black (color discarded) or something else (color saved).