Strange pattern in Ubuntu image viewer


I know this should be posted in some Ubuntu or Debian forum, but I’m just curious if anyone has any idea why this happens.

1 - I open some jpeg in Ubuntu 18.04;
2 - It shows the image without issues;
3 - if I press the TAB key, this strange pattern shows up, until I release the key.

Before pressing TAB:

After pressing TAB key:

Weird. Must be a bug or a hack that should have been removed at release. (Reminds me of videos games where programmers forget to remove convenience code.) Also, there is a spike in your leftmost graph: was that in response to your tabbing?

If I click any control in Image Viewer, the image toggles between the “clean” one and the “dirty” one, and, although the file name at the top of the window continues to say IMG_6284_CR2.jpg, the file name at the bottom of the window toggles between two screenshot png file names.