No support for the face :extend attribute.

[ideasman42]

This comparison shows emacs 29 (current git) with ediff that uses ediff-odd-diff-A face which defaults to:

Face: ediff-odd-diff-A (sample) (customize this face)

Documentation:
Face for highlighting odd-numbered non-current differences in buffer A.

Defined in ‘ediff-init.el’.


           Family: unspecified
          Foundry: unspecified
            Width: unspecified
           Height: unspecified
           Weight: unspecified
            Slant: unspecified
       Foreground: unspecified
DistantForeground: unspecified
       Background: #3e3e5e
        Underline: unspecified
         Overline: unspecified
   Strike-through: unspecified
              Box: unspecified
          Inverse: unspecified
          Stipple: unspecified
             Font: unspecified
          Fontset: unspecified
           Extend: t
          Inherit: unspecified

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Screenshot of emacs/html compared.

[flintforge]

Perhaps solved adding an extra span wherever :extend is true