• Ensure that both sides are passed the appropriate companion data to preserve spacers when syncing
• Remove companion handling in codepaths related to range folding, since this isn't supported in the side-by-side diff
• Move handling of buffer folding into the block map
• Rework set_companion to handle both DisplayMaps at once
• DRY some code around block map syncing in the DisplayMap

TODO:

• diagnose and fix issue that causes balancing blocks not to render properly when they are adjacent to spacers (e.g. merge conflict buttons)
• clear balancing blocks when clearing companion
• additional tests: interaction between spacers and balancing blocks, resizing

Release Notes:

• N/A