Create default cells in newly inherited grid tracks before assigning
added copy children. A cell override previously made sync append an extra
track; a later full sync shrank the tracks beneath the retained cells.
Keep existing authored cells and skip positions covered by existing spans.
Add common and real frontend row/column growth histories, including an
opposite copy direction, retained overrides, exact undo/redo and stable
full sync. The original common test failed 12 of 100 assertions and the
frontend test failed 4 of 100. Keep those assertions and add checks that
each child has exactly one cell.
Sync benchmark lower quantiles (ms), baseline -> now (ratio):
deepest three levels: 35.215 -> 18.770 (0.53)
chain of dependents: 68.931 -> 37.764 (0.55)
library update: 83.083 -> 49.418 (0.59)
main among many shapes: 4.837 -> 3.008 (0.62)
main into 10 copies: 1.790 -> 0.968 (0.54)
main into 100 copies: 15.415 -> 9.474 (0.61)
main into 1000 copies: 153.086 -> 97.063 (0.63)
variant switch: 32.052 -> 22.157 (0.69)
All worlds pass the recorded 10% regression limit. The baseline used
another machine/JVM, so ratios are rough comparisons.
Validation: common JVM 2,016 tests / 33,356 assertions; common JS
1,952 / 32,015; frontend 1,752 / 14,088; backend 1,793 / 9,007;
exporter 16 / 40. Six properties pass 40 programs at seed 712 in
both common runtimes. Lint and format checks pass for all four modules.
Golden-master data is unchanged.
AI-assisted-by: gpt-6
40fc00bcf7