An unglossed root in The Etymologies of the 1930s with derivatives like ᴹQ. panta- “to unfurl, spread out, open” and N. panna- “to open, enlarge”, along with ᴹQ. panta “open” and N. pann “wide” (Ety/PAT). It seems to be a later iteration of ᴱ√PATA² or ᴱ√PṆTṆ from the Qenya Lexicon of the 1910s glossed “open, spread out, show” with very similar derivatives like ᴱQ. panta “open, wide, spreading” and ᴱQ. panta- “open, unfold, spread” (QL/72). In Tolkien’s later writings, ✱√PAT seems to mean “step” or “walk”; see that entry for details.
Neo-Eldarin: Despite the later shift in meaning for √PAT, I think this earlier sense “open (wide)” can be salvaged by assuming that form of the root is actually ✱√PANAT or ✱√PANTA, which is consistent with most of its derivatives. For the non-verbal senses of “open”, I think √LAT is generally better.