[dlfivefifty]

bandwidths(view(A, Block(K), Block(J))) returned subblockbandwidths(A) for any A, ignoring the offset of the block from the diagonal. For a banded (not block-banded) parent with blocked axes, this disagrees with bandeddata of the view, so indexing an off-diagonal block threw:

using BandedMatrices, BlockArrays, BlockBandedMatrices
B = BandedMatrices._BandedMatrix(BlockedArray(randn(5,6), (Base.OneTo(5), blockedrange(1:3))), blockedrange(1:2), 2, 2)
B[Block(2,3)] # BoundsError: attempt to access ... at index [-1:2, BlockSlice(Block(3),4:6)]

For lazy infinite banded matrices, such as products in ClassicalOrthogonalPolynomials/DeRhamOrthogonalPolynomials, A[Block(3,2)] failed instead, because subblockbandwidths is computed from the maximum block size of an infinite axis.

Now, when the parent has an AbstractBandedLayout, the bandwidths are the parent's bandwidths shifted by the block's offset from the diagonal, as for unit-range views in BandedMatrices. DiagonalLayout parents keep the previous behaviour, which the existing tests rely on.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.79%. Comparing base (761b49e) to head (c1f8154).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
+ Coverage   90.76%   90.79%   +0.03%     
==========================================
  Files          11       11              
  Lines        1169     1173       +4     
==========================================
+ Hits         1061     1065       +4     
  Misses        108      108              

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