After login the wallpaper is stretched out for about 1 second. When Plank and Wingpanel are loaded, the wallpaper is changed to the correct scale. This only occurs on a dual monitor setup. How to fix this?
I noticed this happened to me when i change screen resolution. Login screen doesn't adapt to the current resolution, then after login it stretches wallpaper (and rest of the display) to adapt to the correct resolution. Are you using different resolutions that the one used by the login screen?