Emit the document canvas from ImageCompositor for the layer editor

[jtydhr88]

The compositor only returned per-layer refs, fingerprints, and bboxes to the frontend.

When the editor opened without a matching saved state it sized its canvas to the max natural size of the layers, which is wrong for cropped and placed layers (e.g. See-through's decomposed parts): a 1280x1280 document opened as ~855x762.
Worse, saving from that state persisted the wrong canvas back into the widget, so the replayed composite was also clipped.

Report the canvas the composite was actually rendered at via ui[compositor_canvas] (the saved state's canvas on replay, otherwise the document canvas or the placed-layer extent) so the editor can size to it.

Need FE PR Comfy-Org/ComfyUI_frontend#14941

before

2026-08-19.22-50-55.mp4

after

2026-08-19.23-12-04.mp4

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🔇 Additional comments (4)

comfy_extras/nodes_compositor.py (2)

531-533: LGTM!

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549-550: 🗄️ Data Integrity & Integration

Add or identify the frontend consumer for compositor_canvas. This repository contains no tracked frontend reader for this metadata, so the emitted field is unconsumed here. If the consumer is maintained externally, confirm that it uses [{w, h}] only when present and preserves the previous fallback when absent.

tests-unit/comfy_extras_test/compositor_node_test.py (2)

13-13: LGTM!

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74-113: LGTM!

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📝 Walkthrough

Walkthrough

ImageCompositor.execute now restores the saved canvas during replay. It emits compositor_canvas metadata when canvas dimensions are available. Tests cover explicit dimensions, replayed dimensions, and documents without layers.

Merge Risk: ⚪ Minimal · up to 6d44b

This change reports the rendered document canvas so the layer editor can size and save composites correctly. The backend update is localized, and no actionable merge-blocking risk remains beyond the separately tracked frontend follow-up.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: emitting the document canvas from ImageCompositor for the layer editor.
Description check	✅ Passed	The description explains the canvas-sizing defect, the compositor change, replay behavior, and the required frontend follow-up.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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