Any way to flush left context?

[IanDavey]

Hello,

Consider the following toy example:

const replaceStream = require('replacestream');
const { PassThrough } = require('stream');

const input = new PassThrough(), param = new PassThrough(), out = new PassThrough();
input.end('hello{{sep}}world');
param.end(', ');

const replace = replaceStream('{{sep}}', match => {
  replace.pause();
  param.on('end', () => replace.resume());
  param.pipe(out, { end: false });
  return '';
});
input.pipe(replace).pipe(out);

What I was expecting out to contain: hello, world

What out actually contained: , helloworld

Without knowing the internals, my best guess is that the stream before the match is still being held on to when pause is called, making chunks arrive in out out of the expected order. Is there a way to force the left context out beforehand?