message: make consecutive :/@/# separate pill candidates
  e.g. accept the first character always to start the autocompletion
  but after that refer to the plainpart logic to split up in
  new pill candidates. Also, don't allow merging adjacent parts.
  This makes sure that "@nonmatchingfoo
  only matches "someuser" in the autocomplete.
diff: @@ -313,8 +313,16 @@ export class PillCandidatePart extends PlainPart {
  return this._autoCompleteCreator(updateCallback);
  }
  - acceptsInsertion(chr) {
  + acceptsInsertion(chr, i) {
  + if (i === 0) {
  return true;
  + } else {
  + return super.acceptsInsertion(chr, i);
  + }
  + }
  +
  + merge() {
  + return false;
  }
  acceptsRemoval(position, chr) {
  