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Make sure you deselect the Show widget borders checkbox when you edit and paste the code into the Custom HTML widget in Design Studio. :
Check that the widget has been stretched over the appropriate amount of columns and rows in the Design Studio. For example, if you’ve created a widget that is 6 columns wide in the Widget builder, ensure that the Custom HTML widget in Design Studio, where you are pasting the code, is also 6 columns wide.:
Try performing a hard refresh.
If the issue continues, please raise a ticket with Fuse Technical Support.