[tbolis]

Summary

Each rectangle label subscribed to object:scaling and object:moving on the canvas when it was drawn (RectangleLabelObject), and nothing unsubscribed. The two listeners stayed after the label was deleted or the canvas was cleared, so a canvas which is drawn on for long kept more of them, and each moved or scaled object ran all of them.

• The canvas has the two listeners once. They look the label up by the target of the event, in a WeakMap keyed by the rectangle, so a label is not kept alive by them. A WeakSet of canvases tells which ones already have the listeners.
• The listeners are not removed with a label: undo and redo take its rectangle off the canvas and put it back, and the label has to follow it again.
• RectangleLabelObject.update is a method without the event argument, as the target check moved to the lookup.
• CHANGELOG: an entry under Fixed in 1.1.0.

Checks

yarn format:check, yarn lint, yarn typecheck and yarn test pass (450 passing and 1 todo). The 4 new tests are in tools.spec.ts:

• the canvas gets one object:moving and one object:scaling registration for three labels (it failed before the change, with three of each);
• each label follows only its own rectangle;
• the label of a second canvas follows its rectangle (it fails if the "already listening" flag is global, which I checked);
• the label follows after its rectangle is removed and added back.

Not checked: a real mouse drag in the showcase. The tests use a real fabric canvas and fire its events.

Notes for review

• RectangleLabelObject.setText has no callers. It is not touched here.