I save this file as ~/.local/bin/meld

meld

#!/usr/bin/python3
import argparse
import subprocess
import sys
import os
import shlex


def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("-o", "--output", metavar="OUTFILE")
    parser.add_argument("-L", "--label")
    parser.add_argument("--comparison-file")
    args, argv = parser.parse_known_args()

    pre = ["flatpak", "run", "--file-forwarding"]
    post = ["org.gnome.meld"]

    if os.environ.keys() & {"TOOLBOX_PATH", "FLATPAK_ID"}:
        if "TOOLBOX_PATH" in os.environ:
            fxu = "/usr/libexec/flatpak-xdg-utils"
            if fxu not in os.environ["PATH"]:
                os.environ["PATH"] = ":".join((fxu, os.environ["PATH"]))

        pre[0:0] = ["flatpak-spawn", "--host"]

    if args.output:
        post.extend(("@@", args.output, "@@"))

    if args.comparison_file:
        post.extend(("@@", args.comparison_file, "@@"))

    if args.label:
        post.extend(("--label", args.label))

    for arg in argv:
        if arg.startswith("-"):
            post.append(arg)
        else:
            post.extend(("@@", arg, "@@"))

    cmd = pre + post

    sys.stderr.write("$ {}\n".format(" ".join(map(shlex.quote, cmd))))
    sys.stderr.flush()

    os.execvp(cmd[0], cmd)


if __name__ == "__main__":
    main()