LaTeX: adding a footnote to a longtable cell causes table to occupy full width

[jfbu]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

.. list-table:: longtable 1
    :header-rows: 1
    :class: longtable

    * - name
      - description
    * - a
      - b

.. list-table:: longtable 2
    :header-rows: 1
    :class: longtable

    * - name
      - description
    * - a
      - b [#]_


.. [#] footnote in longtable cell

and make latexpdf

Expected behavior
Table retains same width

Environment info

• OS: [e.g. Unix/Linux/Mac/Win/other with version]
• Python version: 3.8.7 (CPython)
• Sphinx version: 3.4
• Sphinx extensions: [e.g. sphinx.ext.autodoc, recommonmark]
• Extra tools: TeXLive 2020

[picnixz]

The issue lies with the LaTeXTranslator:

sphinx/sphinx/writers/latex.py

Lines 1030 to 1031 in b6e6805

	if len(list(node.findall(nodes.paragraph))) >= 2:
	self.table.has_oldproblematic = True

and

sphinx/sphinx/writers/latex.py

Lines 183 to 185 in b6e6805

	elif self.has_oldproblematic:
	return r'{%s*{%d}{\X{1}{%d}%s}}' % (_colsep, self.colcount,
	self.colcount, _colsep) + CR

Because the footnote notes nodes are detected as problematic-like nodes, the table is marked as problematic, causing the usage of \begin{longtable}[c]{*{2}{\X{1}{2}}} instead of \begin{longtable}[c]{ll}.

For the "problematic" detection, we could reject nodes that are nodes.paragraph instances, but not just wrappers for nodes.footnote nodes.

[jfbu]

@picnixz Thanks! I have long felt the problematic handling was problematic (could not resist). Until mid to end-May I am very busy so will probably look at this not before that date.