File System API – Adding text instead of replacing

Say I have a text file with the following content:

AAA
BBB
CCC

And I want to use the File System API to insert a new row in this file, in order to get the following:

AAA
XXX
BBB
CCC

For that I have the following code:

const [fileHandle] = await window.showOpenFilePicker()

const writable = await fileHandle.createWritable({ keepExistingData: true })
await writable.write({
  type: "write",
  position: 4,
  data: "XXX",
})
await writable.close()

However the result is the following:

AAA
XXX
CCC

Is there a way to achieve this without loading the whole content of the file in memory and then overwriting?