[Help] How to ONLY sort part of a file and keep the format?

Hi folks,

As the title. How to sort only the non-TAB-beginning line(s) of a TAB-Based file like the below and keep the format? thanks!
# ------------------------------------------------------------------------------
Origin:
# ------------------------------------------------------------------------------
for openers
want out
a load of old cobblers
want for
        5. one
        6. two
        7. three
muck about
muck around
        20. one
        21. two
        22. three
bottom on
        24. one
        25. two
        26. three
lark around
lark about
darkred
        29. one
        30. two
        31. three
# ------------------------------------------------------------------------------
Expect to see after sorting:
# ------------------------------------------------------------------------------
a load of old cobblers
for openers
want for
want out
        5. one
        6. two
        7. three
muck about
muck around
        20. one
        21. two
        22. three
bottom on
        24. one
        25. two
        26. three
darkred
lark about
lark around
        29. one
        30. two
        31. three
# ------------------------------------------------------------------------------

BRs,
---------
William

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