overleaf/services/web/frontend/js/features/source-editor/extensions/scroll-one-line.ts

import { Command, EditorView } from '@codemirror/view'

function scrollByLine(view: EditorView, lineCount: number) {
  view.scrollDOM.scrollTop += view.defaultLineHeight * lineCount
}

const scrollUpOneLine: Command = (view: EditorView) => {
  scrollByLine(view, -1)
  // Always consume the keypress to prevent the cursor going up a line when the
  // editor is scrolled to the top
  return true
}

const scrollDownOneLine: Command = (view: EditorView) => {
  scrollByLine(view, 1)
  // Always consume the keypress to prevent the cursor going down a line when
  // the editor is scrolled to the bottom
  return true
}

/**
 * Custom keymap for Windows and Linux for scrolling the viewport up/down one line with Ctrl+ArrowUp/Down.
 */
export const scrollOneLineKeymap = [
  {
    linux: 'Ctrl-ArrowUp',
    win: 'Ctrl-ArrowUp',
    run: scrollUpOneLine,
  },
  {
    linux: 'Ctrl-ArrowDown',
    win: 'Ctrl-ArrowDown',
    run: scrollDownOneLine,
  },
]