[boyan01]

Summary

Stacked on #543. Selection geometry moves from one resolver that walks GlobalKey registries and re-implements hit testing per container kind, to text leaves that answer their own geometry. Offsets convert between leaf-local and block display space once, in StructuredBlockSelection.

 SelectableMarkdownBlock (painter unchanged: merge, corner joins, clip, paint order)
-  MarkdownSelectionResolver (2970 lines)
-    resolveList* / resolveQuote* / resolveDetails* / resolveTable* ...
-    walks listItemChildKeys, quoteChildKeys, tableCellKeys, ...
+  MarkdownSelectionLeafScope
+    MarkdownSelectableLeaf(path)          # rectsFor / offsetAt / lineRangeAt / clipRect
+  markdown_selection_geometry.dart       # one container algorithm for list/quote/details/footnote/deflist/table

pointer → container: which leaf? → leaf.offsetAt(local)
        → StructuredBlockSelectionLeaf: local → block display offset (once)
        → controller.setSelection(DocumentPosition)

• StructuredBlockSelection.forBlock is cached per block (Expando) and is the single source for serializer, clamp, and select-all. The controller always delegates to its serializer; the runtimeType fast path is gone, and custom serializer subclasses still work.
• Ported rules: gap between child blocks (midpoint within ±24px horizontal overlap), list marker hit order and prefix rect aligned to the first child's first line, table triple-click selects the cell, code/table viewport clipping.
• AGENTS.md updated; removed the stale TableCellSelection description.

file	before	after
markdown_selection_resolver.dart → markdown_block_keys_registry.dart	2970	38
markdown_descriptor_extractor.dart	1315	738
markdown_block_builder.dart	2372	1828
selection_controller.dart	272	96
new markdown_selectable_leaf.dart	–	782
new markdown_selection_geometry.dart	–	845

Net: −4803 / +2233 lines in lib/.

Commits:

1. pin selection geometry and gestures before the leaf refactor: tests only, on the old implementation.
2. resolve selection geometry from text leaves: the refactor, no test file changed.

Evidence

• Commit 1 adds 44 tests (selection_geometry_snapshot_test.dart: painter rect snapshots within 0.5px for paragraphs, nested lists/quotes, details, deflists, footnotes, tables incl. scrolled-in-quote clipping, scrolled code; selection_gesture_edge_test.dart: gaps, markers, line ends, cross-block drags, blank-space drags, multi-click in nested containers, link taps, copy across unmounted ListView rows). Before: 44/44 pass on the old lib/. After: 44/44 pass on the new lib/ with no test file edited.
• flutter test: 258 passed. flutter analyze: no issues.
• benchmark/selection_geometry_benchmark.dart (avg per selection update): medium 2056 → 2086–2298 µs, large 1846 → 1568–2129 µs; run-to-run noise, no clear regression. incremental_append_benchmark.dart: 468x → 499x, 1533x → 1603x.
• Image goldens were not added: CI runs on ubuntu and they would be platform-dependent; rect snapshots pin painter input instead.

Merge Danger

Door: two-way

Internal only; public API and DocumentPosition semantics unchanged. Revert restores the resolver.

Blast Radius: selection

• Child block GlobalKeys are gone, so a child subtree is not reparented when its structure changes (e.g. a streamed list item going from one child to several); a nested details expanded state or image state may rebuild.
• Container plainText now comes from the structure index instead of descriptors; they agreed in all tested cases.
• Small behavior changes: nested custom codeBlockBuilder becomes a placeholder leaf instead of throwing; empty table cells are no longer hit targets; ragged tables no longer risk a RangeError.
• Selection still rebuilds on change (repaint-only was not attempted: existing tests read SelectableMarkdownBlock.selectionRange after setSelection).
• Needs GUI verification: nested list/quote/details drag, marker area, triple-click units, horizontal scroll clipping, footnote/deflist copy, MixinSelectionArea cross-widget drag, context menu and Cmd+A/C.