fix(xlsx): clip anchored charts/images/drawings to the grid's freeze panes

[Taozizz]

Problem

The XLSX chart, image, and drawing overlays spanned the whole sheet surface with no clipping. Anchored objects are positioned as anchor position − scroll offset, so once the grid scrolled past an object's origin the offset went negative and the object painted over the column/row headers, the formula bar, and the host UI around the viewer:

• Charts/images floated on top of the toolbar and formula bar after scrolling down
• Objects also slid over the frozen panes region, which real Excel never allows

Fix

Clip anchored objects per freeze pane, the way Excel does.

• New components/grid-metrics.ts is the single source for anchored-object geometry:
  • gridBodyLayerStyle insets the overlay layers past the row/column headers
  • createGridPaneLayout() splits the grid body into up to four overflow: hidden sub-panes (main / frozen-rows / frozen-cols / frozen-corner), assigns each object to the pane owning its anchor origin, and compensates scroll only on the scrolling axes
  • Frozen panes stack above the main pane, matching the canvas paint order
• XlsxChartOverlay / XlsxImageLayer / XlsxDrawingLayer become thin views over that module — their three hand-copied versions of the frozen-extent / anchor-rect / scroll-compensation math are deleted
• XlsxGrid now derives its header constants from the same module instead of local literals

Also fixed

GRID_ROW_HEADER_WIDTH in composables/internal.ts said 40 while XlsxGrid actually paints a 48px gutter. The constant only feeds the public rect APIs (getChartRect / setChartRect / getImageRect / setImageRect), which were self-consistent but horizontally offset by 8px from on-screen geometry. Aligned to 48 with internal.ts as the single source. Note: the exported constant's value changes, so hosts that manually compensated for the 8px skew would need to drop that compensation.

Testing

• New tests/component/xlsx-anchored-panes.test.mjs: exact-coordinate assertions that freeze panes split into independently clipped panes, the frozen strip only follows horizontal scroll, and the main pane compensates both axes
• Component suite 81/83 (the 2 failures are pre-existing on master in xlsx-menu-state.test.mjs, unrelated source-regex assertions), XLSX unit tests 29/29, workspace typecheck and vue-xlsx build clean
• Manually verified in the demo (financial-model.xlsx P&L sheet and charts-images.xlsx): charts, images, shapes, and form controls scroll with the cells and clip exactly at the grid-body edge

Before

After

🤖 Generated with Claude Code