static Component selectSubdirOrCreateNew(File motherDir, final VF1<File> action) {
  JList list = jlist(map f2s(listDirs(motherDir)));
  
  onDoubleClickOrEnter(list, voidfunc(S path) {
    callFInNewThread(action, newFile(path));
    disposePIFrame(list);
  });
  
  JButton okButton = listDependentButton(list, "OK", r {
    callFInNewThread(action, newFile(selectedItem(list)));
    disposePIFrame(list);
  });
  
  ret showFrame_returnContents("Choose a directory", centerAndSouthWithMargins(
    withTitle("Please select a directory or create one", list),
    
    westAndEast(okButton, jbutton("Create directory...", r {
      inputText("Name of new directory", voidfunc(S name) {
        File f = mkdirs(newFile(motherDir, name));
        fillListWithStrings(list, map f2s(listDirs(motherDir)));
        selectItem(list, f2s(f));
        focusComponent(okButton);
      });
    }))));
}

Began life as a copy of #1018756