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Move a peg's pivot without moving what hangs off it

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Two bugs in the handles, and the second one meant a peg still had no movable
pivot — which is the complaint pegs exist to answer.

`.peg-grab` was drawn LAST, on top of the four scale corners, and at r=7 against
corners spanning 5.2 to 8.8 from the middle it swallowed the inner half of each
one. It is drawn first now, at r=5, so it stops short of them.

And dragging a peg's cross is a TRANSLATE, not a new pivot. It writes the peg's
`pos`, and a peg is a parent, so everything under it comes along — `peg.mjs` said
so all along: "dragging the peg cross moved its child — dx=18.000". That is the
right behaviour for the drag and no way to say "put the pivot here", so a peg
could be made with its pivot wherever `nest/peg` happened to put it and never
moved again.

⌥-drag now relocates it, which is After Effects' pan-behind split. `nest/repivot`
moves the peg and solves

    pinv(child)' = local'⁻¹ · local · pinv(child)

for each child, because only `local(peg) · pinv(child)` reaches a child, so
preserving that product holds the picture exactly still — verified as a drift of
0, both in `nest-test` and through real ⌥-pointer events in `peg.mjs`. The
child's own channels are never touched, so this works over a measured child,
which is the case the peg exists for.

Refused on a peg whose position is animated, rather than quietly wrong: the
compensation depends on the peg's own transform, so a keyed position wants a
different `pinv` on every frame and one stored matrix is not it. The message says
to put a peg over it instead.

One edit at the end of the drag, not per pointermove: a repivot moves nothing on
screen by construction, so only the cross needs to follow the pointer.

603 CLJS tests, and peg.mjs and onion.mjs pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HkinzDz1VtahZVsujAGRBD
This commit is contained in:
Your Name 2026-10-05 19:25:04 -04:00
parent 00b8ed34ee
commit 357ff1f3b9
6 changed files with 227 additions and 9 deletions
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static/arthur/app.css
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@ -1427,6 +1427,8 @@ button.share-button:hover, button.share-button.on { filter: brightness(1.1); }
.paint-overlay .handles .pivot { stroke: #fff1be; stroke-width: 0.6; pointer-events: none; }
/* A peg draws nothing, so the cross is the only thing to grab it by. */
.paint-overlay .handles .peg-grab { fill: transparent; stroke: none; cursor: move; }
/* While an alt-drag is placing it. */
.paint-overlay .handles .pivot.moving { stroke: #7ad7a0; stroke-width: 0.9; }
.paint-overlay .marquee { fill: rgba(230, 202, 139, 0.12); stroke: #e6ca8b; stroke-width: 0.6; stroke-dasharray: 2 1; pointer-events: none; }
/* The creation-target outline and its name tag. Solid, no handles, and a tag pinned just