What does the Win7 taskbar thumbnail disappear?

If the taskbar thumbnail cannot be displayed,
you can try the following steps:

First right click on the taskbar to open the taskbar properties,
check "“Use Aero peek to preview the desktop" ,
Try if you can display thumbnails?
If it still doesn't work, then:
Open: [Computer]
→[System Properties]
→[Advanced System Properties]< Br>→[Advanced]
→ [Settings] in the first row
Select [Let Win7 choose the best setting] (recommended)
or directly check [Enable Aero Peek]
Last [OK]
If you can't, then the desktop theme will not work,
enter [personalization],
use the theme of the [Aero theme] column.