Stop stale Bubble objects from reacting after Comical replaces them

[JohnThomson]

Problem

Comical.convertBubbleJsonToCanvas registers a new ContainerData for a parent that is already active without stopping the previous generation of Bubble objects from monitoring their content elements. Every time a client turns editing on again for the same canvas (Bloom does so whenever it adds a canvas element), the old bubbles leak and keep running adjustSizeAndPosition on every DOM change under the canvas.

That was mostly invisible until a stale family bubble (a parent or child speech bubble) woke up after its element's level had been renumbered: findRelatives looks the family up in the current bubble list by the bubble's stale cached level, finds nothing, and uniteShapes dereferenced selfAndRelatives[0]:

TypeError: Cannot read properties of undefined (reading 'outline')
    at Bubble.uniteShapes
    at Bubble.adjustSizeAndPosition
    at MutationObserver.<anonymous>

Found while adding a Layer (z-order) submenu to Bloom (BL-15992, BloomBooks/BloomDesktop#8372), which renumbers bubble levels and then calls Comical.update(); it surfaced there as a "A JavaScript error occurred" toast whenever a page had a child bubble.

Fix

• convertBubbleJsonToCanvas stops monitoring for the container it is about to replace, so old bubbles no longer leak.
• A bubble's mutation observer checks Comical.isCurrentBubble(this) (new) and lets go of the content if Comical no longer lists that bubble, instead of redrawing from a stale spec.
• uniteShapes returns when findRelatives comes back empty, as a last line of defence.

Verified by patching the same logic into the Comical module of a running Bloom: the family move that reliably threw before no longer does, and live bubbles still unite and redraw.

🤖 Generated with Claude Code

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This change is 

[greptile-apps]

The implementation appears safe to merge, with the non-blocking concern that its observer lifecycle regression is not protected by an automated test.

Summary

This PR prevents replaced Bubble instances from continuing to react to DOM mutations.

• Disconnects the previous bubble generation before replacing an active container.
• Makes mutation observers release bubbles no longer present in Comical’s current list.
• Avoids dereferencing an empty family lookup as a final defensive measure.
• The behavior is not covered by an automated repeated-conversion regression test.

_{Reviews (1) · Last reviewed commit: "Stop stale Bubble objects from reacting ..."}

[andrew-polk]

@andrew-polk reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on JohnThomson).

[andrew-polk]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on JohnThomson).