When visitors are dragging and dropping website elements, the visual space should remain clear. No selection overlays should disturb their view.
Figure 1: Dragging and dropping main banners on audi.de. 
A case from www.audi.de: When visitors of this website drag and drop the main banners on the primary page, they sometimes unintentionally select content in the neighbouring banners while moving the carousel.
If they don’t release the mouse button at the "right" moment, sometimes the whole forthcoming banner and text becomes selected, which causes distorted view on the banner.
Users should feel relaxed when they are browsing the website. They should not be worried things such as when is the "right" moment to release a mouse button in order to keep the visual space clear.
* Update: The drag & drop issue is now fixed. Congratulations for putting this advice into action.
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