[dotCooCoo]

What and why

Make two artboards side by side, draw a vertical line along the right edge of the second one, and run Object ›
Artboards › Rearrange All Artboards… with Arrange by Row and Change to Right-to-Left Layout. On main both artboards
move and the line stays behind. A line along an artboard's bottom edge stays behind too, and Select › All on Active
Artboard selects neither line.

Rearrange, All on Active Artboard (for objects with no area), Paste in Place's source artboard, Align to Artboard
without an index, Crop Image and its tool, file.place with at, CSS positions, a filter plug-in's artboard and the
Artboard tool decide whether a point is on an artboard with Rect::contains, which leaves out the right and bottom
edges. Document::art_on_artboard counts art flush with those edges (#890).

They now use that rule, edges included, within a millionth of a point (artboard_holds and
Document::artboard_holding). On an edge two artboards share, the artboard to the right of it or below it still holds
the point, except at an end of the edge that lies on that artboard's own bottom or right edge; there the first
artboard holds it. All on Active Artboard now selects a line on a shared edge from either artboard. A press exactly on a right
or bottom edge picks that artboard.

How it was tested

Seven new tests cover the change: tests::artboard_at_counts_the_right_and_bottom_edges (doc), and Rearrange, All on
Active Artboard, Paste in Place, placed type, CSS positions and the Artboard tool's cursor and click in tests_newdoc,
tests_menucmds, tests_clipboard, tests_textimport, css_rules and xform::artboard.

With the call sites reverted to main's containment check, all seven fail. Targeted checks: clippy with -D warnings
on the doc, svg, tools and engine crates, cargo fmt --all -- --check, the doc and tools tests (213 and 376 passed),
css_rules (9 passed), 216 engine tests, and cargo xtask brands, layers and assets.

On Windows 11 (x86-64) with rustc 1.99.0, on top of main at 8bc6532:

• CRAFT_FONTS_DIR=<craft-fonts checkout> cargo xtask ci passes its 9 steps: 7,371 tests passed, 0 failed, 11 ignored.

git merge-tree reports no conflicts with main at 49bdc5c; on that merge, cargo check --workspace --tests, the
xform::artboard tests (13 passed), cargo xtask brands and cargo xtask cursors pass. It conflicts with #2614 on the Rearrange and All on Active
Artboard lines; the resolution keeps #2614's measurement and this branch's edge check. It conflicts with #2592 and #2497
on the Artboard tool's cursor line. #2592's version already calls artboard_at. For #2497, the resolution is
!m.shift && cx.doc.artboard_at(p).is_some().

Checklist

• cargo xtask ci passes (fmt, clippy, tests, assets, brands, layers, wasm).
• No panics in shipped code: no unwrap(), expect(), panic!, unreachable!, todo! or
  unimplemented! outside tests, and no unchecked indexing on data from files, parameters or
  messages. Errors are returned as Result and shown to the user
  (why and how).
• Clean-room: nothing copied from Adobe products or GPL/AGPL code (AGENTS.md).
• Every new asset has a row in ASSETS.md.

[bflatastic]

Thanks @dotCooCoo, merged, including your doc-comment and shared-corner test commit. It met #2614 in Rearrange and Select All on Active Artboard: symbol instances are measured by their art and the artboard is found with your artboard_holding / artboard_holds. #2665 now uses the same helpers instead of its own copies. It went through the full gate (fmt, clippy, tests, layers, wasm) in a batch with the other PRs landed today.