[tbolis]

Summary

Adds the polygon tool for 1.1.0 (issue #96): tool="polygon", Tools.Polygon, Types.Tool.POLYGON.

• Each click or tap places a vertex where the pointer is released. A line follows the pointer from the last vertex (on touch, while the finger is down, so the vertex lands where the finger is lifted).
• A polygon is closed by a click or tap on the first vertex (within 10 pixels, 20 on touch, the first vertex is ringed when the pointer is near enough), by a double click or double tap, or by the Enter key. The Escape key cancels it. It needs three vertices, with fewer the closing gestures do nothing. The keys are listened to on the document only while a polygon is in progress, and not when the focus is in a field to type in.
• The polygon in progress is drawn on the top canvas only, as the laser pointer is. It is not an object, not in toJSON() or toDataURL(), and not in the undo history. When it is closed it is one fabric.Polygon and one undo step, onObjectAdded is called once with it, and onChange is called once, not for each vertex. onMouseDown and onMouseUp are still called for every click.
• It takes lineColor, lineWidth and fillColor (transparent by default) as the rectangle does. It is a shape like the others: the select tool moves, scales and rotates it as a whole, the eraser cuts it, and toJSON() and fromJSON() save and load it. The README says how to read its vertices from points.
• Another tool closes a polygon with three vertices and keeps it, and drops one with fewer. Unmounting drops it.
• The vertices are in canvas coordinates, and the radii are in screen pixels, so a change of the view of the canvas does not disturb a polygon in progress. zoom() and the resize of the canvas scale the polygon in progress with the objects.
• Showcase: a Polygon button with a one-line hint, and the full instructions in its tooltip. README: tool table and a section. CHANGELOG: 1.1.0 entry.
• The tools get three hooks: finish() (called before dispose() when another tool takes over), scale() (called when the sketch scales its objects) and notifiesChange(), which replaces the check of the laser pointer in the events manager. autoresize() returns the factors it scaled by.

Checks

yarn format:check, yarn lint, yarn typecheck and yarn build pass. yarn test: 334 passing and 1 todo, on React 19 and on React 18 (58 tests for the polygon). Changing the code on purpose made the tests fail in each case I tried (the closing gestures, the radius in screen pixels, the eraser mark, the transforms of the select tool, the export, the scaling).

In headless Chrome, driven through the DevTools protocol with real mouse and touch events: the closing gestures, the preview and the ring, Escape and Enter, a polygon closed by touch, the export with a polygon in progress (identical to the export of the empty sketch), a resize of the window in the middle of a polygon, and the hint on one line at every window width from 320 to 1920 pixels.

Not checked: a physical touch device, and browsers other than Chrome.

Notes for review

• Closing keywords do not act on a merge into release/1.1.0, so Draw Polygons? #96 needs to be in the Fixes line of the release PR (Release v1.1.0 #196).
• Pinch zoom (It's very cool!! but it doesn't support multitouch on mobile device #84) is a separate PR, built on this branch.
• A polygon which crosses itself is drawn as it is. Editing the vertices of a polygon which is closed, and open polylines (FEAT: PolyLine #72), are not part of this.