helix lf

config.toml

[keys.normal]
C-f = [":new", ":insert-output lf-pick", "split_selection_on_newline", "goto_file", "goto_last_modification", "goto_last_modified_file", ":buffer-close!", ":theme nord", ":theme default"]
# replace the default after theme with the theme you use
# open 1 with the open command (l and <left> to open) or more with (<space> to select) then quit
# all opened files will be opened in helix

lf-pick

# touch ~/.local/bin/lf-pick
# chmod +x ~/.local/bin/lf-pick
function lfp(){
  local TEMP=$(mktemp)
  lf -selection-path=$TEMP
  echo >> $TEMP
  while read -r line
  do
    echo "$line"
  done < "$TEMP"
}

lfp

lf -selection-path=/dev/stdout

lf -print-selection works also. Additionally instead of theme switching :redraw solves UI re-rendering.

[keys.normal]
C-f = [":new", ":insert-output lf -selection-path=/dev/stdout", "split_selection_on_newline", "goto_file", "goto_last_modification", "goto_last_modified_file", ":buffer-close!", ":redraw"]

Thank you for this.
The best version of this I've found so far (from scratching around in all the various answers), is:

C-f = [":new", ":insert-output lf -print-selection", "split_selection_on_newline", "goto_file", "goto_last_modification", "goto_last_modified_file", ":buffer-close!", ":redraw"]

With no lf-pick script needed.

It almost works too! But you need to have modified a buffer to get it to open, and it doesn't open from the current location, which is a pain.

I think it's a testament to how much helix needs this functionality that we're all scrabbling around in here, trying desperately to make something work...