[michaelstonis]

Closes #26.

What

• RecordingDrawingContext (tests/KumikoUI.Core.Tests/TestHelpers): records every IDrawingContext call as a DrawOp: kind, viewport geometry (translations applied), copied paint, call index (z-order), and the intersected clip in effect. DrawOp.Visible is what the clip lets reach the screen. Text measures monospace (8 px per char, 14 px tall). It also tracks Save/Restore balance.
• RenderHarness: a four-column grid of people wired to DataGridRenderer. Every GridColor on the style gets a distinct value, so a recorded colour identifies its style role.
• 79 renderer tests (96 cases) in tests/KumikoUI.Core.Tests/Rendering/, covering every Draw* method:
  • frame clear and Save/Restore balance across 10 configurations
  • header: titles, reserved icon space, per-column header style, z-order over scrolled rows
  • rows: visible range, placement, alignment, scrolling, alternate/selected/focused backgrounds, row and cell style resolvers, column cell style, edited cell skipped, custom and replaced cell renderers, Boolean checkbox
  • grid lines, current-cell border, sort indicators with multi-sort badges, filter icons (inactive and active)
  • left-frozen, right-frozen and scrollable passes: positions, clips, backgrounds, dividers, and no scrollable cell reaching the frozen pane
  • frozen rows: fixed position, z-order, styles, divider; the drag-handle column on both sides
  • grouping panel (label, chips, sort arrow, hierarchy chevron); group header rows (text, count, caption, chevron per expansion, nesting indent); group summary rows (including overflow and untitled)
  • top and narrow-column overflow table summaries
  • column and row drag overlays, including drops into the frozen band
  • editor and popups drawn last and unclipped
• DataGridRenderer line coverage: 99.7% (branches 92.9%). The 3 lines left are the CORE-03 bug line, an unreachable group-row guard in frozen rows, and a catch block.

Known defects pinned (acceptance criterion: CORE-03/CORE-25 caught)

Three [Fact(Skip = "…, fixed by #NN")] tests. I checked that each one fails on current code when un-skipped:

• CORE-03 → Style resolution must not mutate shared CellStyle instances #22: a row-style text colour leaks into the column's shared CellStyle, so row P01 turns red.
• CORE-25 → Clip summary values to their pane; horizontal grid lines on frozen rows #27: table and group summary values of a scrolled column overlap the frozen pane by 42 px.

Each fix should remove its Skip.

Source changes

• Removed the dead private DrawFrozenBackground and the unused ColumnScreenX(bool) overload. No behaviour change.
• GridRect.ToString() now returns GridRect(x, y, w, h) (invariant culture), so assertion failures are readable.
• .github/instructions/testing.instructions.md: the section describing a FakeDrawingContext that never existed now describes RecordingDrawingContext/RenderHarness (TEST-09; the full rewrite stays in Internal docs regenerated from code: ARCHITECTURE, RENDERING, Copilot instructions and prompts, testing instructions, Uno instructions; docs/uno becomes a user guide #96).
• PinnedBottomSummaryTests adapted to the recorder's richer API.

Verification

• Core tests: 465 passed, 3 skipped (main: 372).

🤖 Generated with Claude Code

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