Sample single line setup CodeMirror; note that you need to tweak the default keymap

cm-single-line.js

import { minimalSetup } from 'codemirror'
import { EditorState } from '@codemirror/state'
import { EditorView, keymap } from '@codemirror/view'
import { insertNewlineAndIndent, insertBlankLine } from '@codemirror/commands'

function removeNewlineCommands(exts) {
  const newlineCommands = [insertNewlineAndIndent, insertBlankLine]
  
  return exts.map(ext => {
    if (ext?.facet == keymap) {
      const newMap = ext.value.map(cmd => 
        newlineCommands.includes(cmd.run) ? 
          { key: cmd.key, run: () => true } : cmd)
      // shift-enter is not registered in the keymap, and it replaces selections 
      // with newline chars; A tx causes by it should be ignored, but I do not 
      // know how to do that when it occurs between two txs.
      // XXX: will not be needed if this patch is released:
      // https://github.com/codemirror/commands/commit/22d2119
      return keymap.of([...newMap, { key: 'Shift-Enter', run: () => true }])
    }
    return ext
  })
}

function dispatchTransactions(trs, view) {
  const docChanged = trs.some(tr => tr.docChanged)
  if (!docChanged) {
    view.update(trs)
    return
  }

  const lastTr = trs[trs.length - 1]
    
  const removeNLs = []
  for (let len = 0, iter = lastTr.newDoc.iterLines().next();
       !iter.done;
       len += iter.value.length + 1, iter.next()) {
    // skip the first line
    if (len == 0) continue
    removeNLs.push({from: len - 1, to: len})
  }

  console.log(removeNLs)
  view.update(removeNLs.length ? 
              [...trs, lastTr.state.update({ changes: removeNLs })] :
              trs)
}

new EditorView({
  doc: 'this editor only allows a single line',
  extensions: [
  removeNewlineCommands(minimalSetup),
    EditorView.lineWrapping,
    EditorState.allowMultipleSelections.of(true),
  ],
  dispatchTransactions,
  parent: document.body,
})