Not idempotent: nested multi-line tags oscillate between two indentations forever (no fixed point exists)

[PetrDlouhy]

Summary

For certain well-formed HTML, djhtml never converges. It flips the indentation
of a nested multi-line open tag back and forth between two states on every run,
indefinitely — a deterministic 2-cycle.

Worse than "runs twice": neither state is accepted by djhtml -c. Both
states report would have reindented, so there is no content you can commit
that passes the check. In a pre-commit/CI setup the gate fails permanently, and
an autoformatting bot ping-pongs commits.

System information

• djhtml 3.0.11 (installed from PyPI into a clean virtualenv)
• Also reproduces on 3.0.10
• Python 3.12.3, Linux 6.8
• Default settings — no arguments, no config file, run in an empty directory

Minimal reproduction

repro.html — plain HTML, no Django/Jinja template tags involved:

<form action="/x"
      method="post">
    <button type="submit"
            class="btn">Pay</button>
</form>

$ md5sum repro.html
9744086f3c8484895d859bc02aa3ba77  repro.html

$ djhtml repro.html && md5sum repro.html
reindented repro.html
1 template has been reindented.
45937171220590e694a12aa2949f16a3  repro.html

$ djhtml repro.html && md5sum repro.html
reindented repro.html
1 template has been reindented.
9744086f3c8484895d859bc02aa3ba77  repro.html      # <- back to the original

$ djhtml repro.html && md5sum repro.html
reindented repro.html
1 template has been reindented.
45937171220590e694a12aa2949f16a3  repro.html

The checksums alternate A, B, A, B, … — verified over 6+ iterations. The diff
of a single run:

 <form action="/x"
       method="post">
-    <button type="submit"
-            class="btn">Pay</button>
+  <button type="submit"
+          class="btn">Pay</button>
 </form>

No fixed point exists

Both members of the cycle fail the check:

$ djhtml -c state_A.html
would have reindented state_A.html
1 template would have been reindented.        # exit 1

$ djhtml -c state_B.html
would have reindented state_B.html
1 template would have been reindented.        # exit 1

So djhtml -c cannot be satisfied for this file by any amount of reformatting.

What triggers it

Both the parent and the child must have multi-line open tags (a single-line
child is stable). Whether it oscillates then depends on the column at which the
parent's attributes continue — i.e. on the length of the parent tag name.

Sweeping parent tag-name length with the child at indent 4:

parent tag length	attribute continuation column	result
1–3	3–5	stable
4	6	oscillates
5–7	7–9	stable
8	10	oscillates
9	11	stable
10	12	oscillates
11–12	13–14	stable

In practice this means common 4-letter tags — <form>, <span>, <main> —
oscillate, while <div> does not, which is why it can hide for a long time and
then appear the moment a template is wrapped in a <form>.

Related, but not duplicates

• Unclosed attribute values cause superfluous indentation on subsequent runs #118 (Unclosed attribute values cause superfluous indentation on subsequent
  runs, closed) — also multi-pass instability, but it is caused by malformed
  HTML (an unclosed quote) and it does converge after N passes. This report is
  well-formed HTML that never converges.
• Avoid modifying indentation of multiline attributes #92 (Avoid modifying indentation of multiline attributes, closed) — related
  area, different request.

I could not find an open or closed issue about non-idempotency / oscillation on
well-formed input.

Impact for us

This is the sole remaining reason one of our templates has to stay on the
formatter's exclusion list: after djLint formats it into exactly this nested
multi-line shape, our djhtml pre-commit gate passes or fails depending on run
parity.