Backport candidates for ITK 5.4.6 release #6051

Tracking issue for commits on `main` that should be considered for backporting to the `release-5.4` branch for ITK 5.4.6. See also Discourse: Patches for ITK 5.4.6.

Open PRs targeting release-5.4

• Release 5.4 update gdcm #6059 BUG: Backport GDCM CVE-2026-3650 fix #6078 Release 5.4 update GDCM (thewtex) — addresses CVE-2026-3650 security issue
• vtk field for release #4859 BUG: VTKPolyDataMeshIO FIELD data support (thewtex) — rebased, code cleaned up
• BUG: Backport null checks after ReadHeader in IPLCommonImageIO #6064 BUG: Backport null checks after ReadHeader in IPLCommonImageIO (Tier 1)
• BUG: Backport value-initialize singleton instances #6065 BUG: Backport value-initialize singleton instances (Tier 1)
• BUG: Backport ImageAdaptor::ComputeOffset for ImageRegionIterator #6066 BUG: Backport ImageAdaptor::ComputeOffset for ImageRegionIterator (Tier 2)
• BUG: Backport zero-check guard in CumulativeGaussianCostFunction #6067 BUG: Backport zero-check guard in CumulativeGaussianCostFunction (Tier 1)
• COMP: Backport IO modules publicly depend on ImageIOBase #6068 COMP: Backport IO modules publicly depend on ImageIOBase (Tier 3) — closed; correctness fix without reported breakage, out of scope for patch release

Already merged to release-5.4

• DOC: Sync spell-check dictionary with main branch #6056 DOC: Sync spell-check dictionary with main branch
• BUG: Backport BresenhamLine integer-only algorithm and double precision fix #6054 BUG: Backport BresenhamLine integer-only algorithm and double precision fix
• COMP: Update retired windows-2019 CI runner to windows-2022 #6053 COMP: Update retired windows-2019 CI runner to windows-2022
• BUG: Backport Voronoi infinite loop fix to release-5.4 #6052 BUG: Backport Voronoi infinite loop fix
• ENH: Backport #6027 — comprehensive tests for itk.Image buffer protocol and lifetime #6043 ENH: Backport comprehensive tests for `itk.Image` buffer protocol and lifetime
• ENH: Backport #6026 — PEP 688 buffer protocol and np.asarray() lifetime safety #6042 ENH: Backport PEP 688 buffer protocol and `np.asarray()` lifetime safety
• COMP: Move inline = default destructors to .cxx for 30 exported classes #6041 COMP: Move inline `= default` destructors to `.cxx` for 30 exported classes
• ENH: Add ITK_PYTHON_RELEASE_GIL option and SWIG -threads flag #6022 ENH: Add `ITK_PYTHON_RELEASE_GIL` option and SWIG `-threads` flag
• Backport #6007 For FFTW feature selections to release-5.4 #6025 Backport FFTW feature selections from PERF: Use ABI-guaranteed SIMD baselines for redistribution-safe FFTW builds #6007 (closed)

Tier 1 — Critical bug fixes

• `0eb1c42d` BUG: Add null checks after ReadHeader in IPLCommonImageIO → BUG: Backport null checks after ReadHeader in IPLCommonImageIO #6064
• `2f37d7a3` BUG: Value-initialize singleton instances → BUG: Backport value-initialize singleton instances #6065
• `d7845fee` BUG: Reorder zero-check guard in CumulativeGaussianCostFunction → BUG: Backport zero-check guard in CumulativeGaussianCostFunction #6067
• RECOMMEND REJECT `2eb511f4` BUG: Fix TIFF palette allocation — cherry-pick conflicts with release-5.4 style divergence in itkTIFFImageIO.cxx; manual adaptation required
• RECOMMEND REJECT `381bafa4` + `3fe861f0` BUG: Fix Math::Absolute — GTest file deleted on release-5.4, itkMath.h heavily refactored; high conflict risk

Tier 2 — High-value fixes

• `1e3a8833` BUG: Add ImageAdaptor::ComputeOffset → BUG: Backport ImageAdaptor::ComputeOffset for ImageRegionIterator #6066 (test adapted for release-5.4: no CTAD, no MakeIndexRange)
• RECOMMEND REJECT `c3becd3a` BUG: QuadEdgeMesh past-the-end pointer — cherry-pick conflicts with test file refactoring on main

Tier 3 — Build fixes

• REJECT `12be7405` COMP: VTKImageIO publicly depends on ImageIOBase → COMP: Backport IO modules publicly depend on ImageIOBase #6068 (closed) — correctness fix, no reported downstream breakage on release-5.4
• `3b3c1e15` COMP: Add missing typename in GPUDemonsRegistrationFunction — already present on release-5.4
• RECOMMEND REJECT `bfc8e07d` COMP: Promote ITK header paths for name mangling expat.h — rejected previously
• RECOMMEND REJECT `fc45cd9b` + `1d0305e7` COMP: Fix Python3::Module target not found on Windows — needs investigation; 2-commit dependency chain touching CMake infrastructure

Rejected for backport

• REJECT COMP: Backport IO modules publicly depend on ImageIOBase #6068 COMP: IO modules publicly depend on ImageIOBase — correctness fix, no reported breakage; fix going to main instead
• REJECT COMP: Remove hardcoded -march=corei7, -mtune=generic, and dead code #6049 COMP: Remove hardcoded `-march=corei7` — may not apply cleanly
• REJECT COMP: Fix ccache hit rate on Azure DevOps to near-zero #6048 COMP: Fix ccache hit rate — different infrastructure on release-5.4
• REJECT `bfc8e07d` COMP: Promote ITK header paths for expat.h — previously rejected

Methodology: Compared `upstream/release-5.4..upstream/main` (2450 commits: 164 BUG, 232 COMP, 39 PERF). Filtered to commits after 2025-12-01 with ≤3 files changed for minimal backport risk. Excluded test-only and STYLE changes.