Zooming out with a Magic Wand selection freezes the app (marching ants redraw scales with outline size)

[MyronKoch]

What happens

After a Magic Wand selection on detailed artwork, zooming out freezes the app. The main thread stays pegged at 100% and never recovers. I hit it on a 27,000 x 5,400 banner at 0.6% zoom, and it also reproduces on unmodified main with a 10,800 x 5,400 PSD.

Steps

1. Open a document with detailed artwork (ornate lettering, logos).
2. Click the Magic Wand on the artwork with default settings (tolerance 32, contiguous).
3. Zoom out. At 10% it's already sluggish; at around 1% it stops responding.

Measurements

These were taken on main at 01e8e52 from a local test. It ran MagicWand.select on a real 10,800 x 5,400 PSD, then timed exactly what TransformOverlay.drawSelection() does: a solid 1 pt stroke, then a [4, 4] dashed stroke, into a view-sized context.

The wand's outline size depends on what's clicked:

Clicked on	Outline edges
Flat background	4
Schedule text layer	26 to 52,006
Poster artwork layers	194,548 to 271,662

Drawing the largest outline (271,662 edges) once:

Zoom	Solid	Solid + dashed
100%	0.027 s	0.055 s
10%	2.68 s	3.27 s
0.6%	63.9 s	94.9 s

The ants timer (EditorCanvas.swift:2008) marks the overlay dirty every 0.12 s. From about 10% zoom down, a single redraw takes longer than that interval, so the next one is already queued and the app never catches up. The cost rises as you zoom out because every edge is still processed while hundreds of thousands of them collapse into a few screen pixels. The time is spent inside Core Graphics, so a Release build doesn't change it.

A sample of the frozen app shows the main thread in a single CA::Layer::display → CGDisplayListDrawInContextDelegate → ripc_DrawPath → aa_render → aa_distribute_edges call, identical across every sample for minutes.

The wand allows outlines of up to 8,000,000 edges (WandPixels.c:7, wand_edge_limit), so an outline far larger than the one measured here can reach the overlay.

Where

• Compositor/Rendering/TransformOverlay.swift:197, drawSelection(): strokes the full selection.path twice per redraw, at full detail, at any zoom.
• Compositor/Rendering/EditorCanvas.swift:2008: the 0.12 s marching-ants timer.

Possible fixes

1. Level of detail for the outline. When a document pixel is smaller than a screen point, draw the ants from an outline traced at screen resolution instead of per document pixel. Tracing the selection mask downsampled to the current zoom, or caching one outline per zoom step, keeps the edge count bounded by what's on screen. This is the real fix.
2. Backpressure on the ants timer. Skip a tick while the previous redraw is still pending, so a slow outline stutters instead of freezing the app. It's small and independent of fix 1, and worth having in any case.

I can send a PR for fix 2 if you'd like it, or try fix 1 if you have a preferred approach.

Environment: macOS 27.0 (26A428), Apple M3 Max, Compositor main at 01e8e52.

🤖 Generated with Claude Code

[robbietilton]

Thanks for the detailed report. We couldn't reproduce the freeze ourselves — synthetic outlines of ~500k edges drew in about 0.04 s at every zoom here, so it seems to depend on the shape of your artwork. But we believe we have a fix in place, pushed to main in 8eba077:

• Level of detail: below 1:1, complex outlines (over 20k path elements) are traced at screen resolution in the background and cached per zoom step, so the ants never draw more edges than the screen can show. Marquees, lassos and anything at 100% or closer draw exactly as before.
• Backpressure: the ants timer skips a tick while the previous redraw is still pending, so a slow outline stutters instead of freezing.

It'll ship in 1.2.10, or you can build from main now. Would appreciate it if you could re-test with your banner and let us know.

[MyronKoch]

Thank you, that was fast. Re-tested on Compositor 1.2.10 (build 25), which includes 8eba077, with the same 27,000 x 5,400 banner that froze before. I made a Magic Wand selection around the ornate artwork (tolerance 32, Add, This Layer) and zoomed out to 2.9%. The ants draw, and the app stays completely responsive.

On the reproduction gap: the outline I measured came from real artwork, 271,662 edges on a 9,998 x 15,441 layer, so its shape may well be what synthetic outlines don't capture. If it's useful, I'm happy to share the small test I used to time it.

From my side this is fixed, so feel free to close. Thanks again.

[robbietilton]

Thanks for re-testing. Closing.