Remove panel svg shadow without removing blur

[luisbocanegra]

Duplicate shadow:

Hiding the default background also removes the blur, setting its opacity doesn't hide the shadow, so an option that hides just the shadow is needed.

[rileyaft]

Additionally, as hiding the default background hides the blur - an option to re-enable blur would be greatly appreciated for when custom coloring is applied.

Picture shown below to compare blur between transparent panel and blurred dolphin side-panel view.

(Would have created new bug, but this is already incredibly similar)

[luisbocanegra]

Additionally, as hiding the default background hides the blur - an option to re-enable blur would be greatly appreciated for when custom coloring is applied.

Hiding the panel background removes the blur, contrast, and shadow as they depend on the panel svg from the Plasma Style.

This is why the opacity was added, setting it to 0 does the trick (hiding unchecked), but since the shadow doesn't depend on the opacity it is still drawn, this what this issue is about.

Also note that in the main branch is now possible to use the custom panel background #50 or widget backgrounds #53 as the blur/contrast source (Original background hiding unchecked and with opacity set to 0):

[luisbocanegra]

Possible alternative, removing the shadows from the theme SVGs seems to work fine and does not block us from drawing blur: