ZipperMesh silent failure in one-cell-wide 2D overset problems

Description

For overset cases with overlapping surface meshes (such as combined internal/external flow problems where an integration zone is required), the zipper mesh routine can fail to produce a zipper mesh for certain meshes that are one cell wide without raising an error message. I have observed multiple meshes where , e.g., 3 of the 4 overlap regions produce good zipper mesh cells but one has a gap.

Steps to reproduce issue

Placeholder, need to produce an open source-able test case

Current behavior

Zipper mesh debug info showed "Starting String ...." with no further debug info printed for that string (some kind of early exit is happening)

Zipper mesh successfully created for same mesh if I extrude two cells wide instead of just one.

Expected behavior

Zipper mesh created for each overlap region

Code versions

• Operating System: Amazon Linux, WSL2
• This repository: 2.5 (february 2022 following Sabet's zipper mesh segfault fix)

Start with the zipper mesh routine and its debug output, especially the path that prints "Starting String" without further information. Create an open-sourceable one-cell-wide 2D overset test case, compare it with the two-cell-wide version that succeeds, and verify that every overlap region produces zipper mesh cells without a gap.