Update chunk to an empty document leaves change unchanged

[Vinzent03]

Describe the issue

I noticed while using the merge package in my Obsidian plugin that when updating an existing chunk via updateB on, which reverts any changes to an empty document, the old chunk is still preserved. This results in a chunk referring to positions that no longer exist. It can be reproduced by the following test. It does not just apply to a single character, but if one document is empty and the other other is empty after the change as well.

  it("properly handles changes to empty doc", () => {
    let sA = EditorState.create({doc: ``})
    let sB = EditorState.create({doc: `a`})
    let chs = Chunk.build(sA.doc, sB.doc)
    ist(chs.length, 1)
    // Remove the single character 'a' from sB, which should clear the single chunk
    let tr = sB.update({changes: {from: 0, to: 1,}})
    let updated = Chunk.updateB(chs, sA.doc, tr.newDoc, tr.changes)
    ist(updated.length, 0)
  })

This is a regression by a fix to my last issue report: #1634 codemirror/merge@fe6b1ad. v6.11.1 does not have this issue in my plugin.

Browser and platform

Observed in Obsidian, Electron, but reproduce as well in nodejs v24.11.0

Reproduction link

No response

[marijnh]

Thanks for the test case. Attached patch seems like the more correct way to formulate that condition.