**A frames, groups, sections and masks behaviour suite written from the public documentation, and fixes for every divergence it found.** Each documented rule (frames vs. groups when resized, constraints, Cmd/Ctrl-resize, clip content, Frame selection, grouping and ungrouping, group bounds and opacity, X/Y in groups, click targets, drawing a frame or section around layers, dropping into and out of frames, where sections may go, resizing sections, top-level frames as screens and exports, Use as mask, what a mask covers, alpha / vector / luminance masks, groups as masks, clicks on masked-out areas, a mask group's box and exports) is a table-driven case with the result the rule implies and a one-line derivation; where the documentation is silent the case says so and pins today's behaviour.
**Fixes (the suite failed on 7 tests before them)**
- **Masked-out areas don't take clicks**: hit testing and Select layer skip what a visible mask hides (the nearest mask below a layer; for a group or frame used as a mask, what its layers draw), so a click there reaches what's behind. Checked only once something is hit, and skipped in files without a mask (`Document::has_masks`, worked out once after each change), so hit testing costs what it did.
- **A mask group's box and exports are the mask's**: a group's bounds and export area count a mask for the layers it masks (layers under the mask still count), so the selection box, W/H, snapping and PNG/SVG/PDF exports crop to the mask instead of the whole masked layer (`Document::box_children`).
- **Sections stay on the page or in sections**: moving a section into a frame or group (`node.reparent`, refused at the root in `Document::reparent`), dragging one over a frame (it lands in the section around the frame, or on the page), grouping or framing a section, creating a section in a frame and pasting one into a selected frame (it goes on the page) no longer put sections where they can't be.
- **Drawing a frame or section around layers puts them in it**: the layers fully inside the drawn box (its siblings; visible and unlocked; a frame never takes a section; not in auto layout) move into it, in their order, without moving on the page; one undo step.
- **Resizing a section leaves its contents** where they are on the page (no constraints in sections), from a field, a handle (also the left/top edges) or `transform.resize` with any anchor.
- **Cmd/Ctrl while resizing a frame leaves its layers as they are**: #133 (merged in) keeps their places; this branch also keeps their sizes (the frame is resized without running constraints, so a left & right or scale child no longer stretches and is only moved back).
- **X/Y are measured in the frame, not the group**: groups and boolean groups have no coordinate space of their own, so a layer in a group shows (Design panel, `node.get`, `inspect_document`) and takes (`node.set`, `node.create` with a group parent) X/Y in the frame the group is in, or on the page (`Document::frame_relative`). Rotated groups too.
Already there and now covered by the suite: constraints when a frame resizes, groups scaling their layers, clip content (drawing, hits, export size), Frame selection's size and place, grouping/ungrouping keeping world transforms (rotated groups too), nested groups, empty groups going away, group opacity on the composite, click / Cmd-click / double-click targets (frames in sections count as top-level frames), dropping by the pointer and pulling out, group layers staying in their group when dragged, top-level frames as prototype screens and export boards, Use as mask (bottom layer, "Mask group", at the topmost layer's place, single-layer toggle), masks covering every layer above them, hidden masks, alpha / vector / luminance masks, a group as a mask, masks in SVG and PDF exports.
No new commands or UI strings (no i18n changes); each edit stays one undo step.
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