Swap 2 box shadows for 1 radial gradient

Closes #4340

Fixes a UI glitch where the vertical parts of a box shadow would show.
This fix changes those box shadows for 1 radial gradient, where only 5px of it is shown. No more vertical shadows.
Merge pull request #4341 from PaulAdamDavis/fix-floating-scroll-shadows

Fix floating header vertical shadows glitch