Discovered while testing MC-269937.
The bug
There does not appear to be a texture file associated with the translucent, whitish overlay displayed on top of a world's thumbnail when the cursor is placed on top of a world.
How to fix
Introduce a proper texture file for this effect.
Linked issues
relates to 18
MC-278413
Chat background cannot be retextured
Open
MC-264571
Both standing and wall sign GUI backgrounds cannot be retextured independantly from the block
Resolved
MC-270255
Item slot disabled effect cannot be retextured
Open
MC-270258
Durability bar cannot be retextured
Open
MC-270260
Bundle fullness bar cannot be retextured
Open
13 more links
Can confirm