The cm-codeblock line decoration carried vertical margin, per-line padding, and border-radius, so each code line rendered as its own rounded box with the editor background showing through the gaps between lines. Drop the vertical margin and give the block horizontal padding only, then tag the first/last block lines (cm-codeblock-first / -last) to add the vertical padding and rounded corners — the run now reads as one continuous box. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>