[matthewalberto]

The basics

• I validated my changes

The details

Resolves

Fixes #10675.

Proposed Changes

Prevent duplicate blocks from triggering unintended stack adjustments during their initial render.

Defer bumping other blocks until after the duplicate has been positioned.

Preserve the initial render step so Blockly can calculate the duplicate's connection positions correctly.

Add regression tests covering duplication of nested blocks with children.

Reason for Changes

When duplicating a block nested inside another block, the duplicate initially appears directly on top of the original. Blockly is expected to move the duplicate down and to the right.

Previously, Blockly performed bumping during the initial render, before repositioning the duplicate. Because the duplicate's edges aligned with the original block's interior, the original stack was bumped instead, leaving the duplicate in its initial position.

Deferring bumping until after repositioning ensures that the correct blocks are moved.

Test Coverage

Full test suite: Added 4 regression tests. The original implementation fails 3 of the 4 new tests and passes 4044 tests overall. With this fix, all 4047 tests pass.

Mutation testing: All 7 deliberate breakages were caught. An additional test was added after one breakage escaped the initial run.

Lint and formatting: Clean on all 3 changed files. The only failure in the wider lint run is caused by Windows line endings in unrelated files; CI runs on Linux.

Not tested: Real-browser behavior, Linux locally, and alternative renderers.

Documentation

No documentation changes are needed.

Additional Information

Claude Code (Claude Opus 5.5) was used collaboratively throughout the process, including reproducing the bug, implementing the fix, writing the four regression tests, and performing validation checks. All PR description text was written independently by the human contributor.

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