Layers: the shown document's thumbnails are not held up by another document's queue

The Layers and Links panels share one thumbnail thread across documents,
and it ran its jobs in the order they came. Rows of a document left with
a long queue held up the thumbnails of the next document shown.

The thumbnail thread can now drop the jobs it has not started for some
slots. Each frame the Layers and Links panels name the document they
show; when it changes, the queued jobs of every other document are
dropped, and when no document is open, all of them are. Those rows ask
for their thumbnails again when their document shows again, and the
thumbnails already made are kept. The Navigator and the arrange panes do
not use the new call.

With 150 rows of 20,000-anchor paths queued for one document, the 8 rows
of such paths of the next document shown get their thumbnails in 153 ms
instead of 3,265 ms (debug test build, median of three runs).

docs/target-app-parity.md

@@ -1289,6 +1289,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   In the web build the status bar's render and UI times are measured with the browser's `performance.now()`, and the Layers panel stops rendering row thumbnails in a frame once they have taken 8 ms.
   On the desktop, the Layers panel renders a row's thumbnail on its thumbnail thread when the row's paths, and the paths of its opacity masks that are not disabled, hold more than 2,000 anchors in all. It already did this for the first thumbnail of a row of more than 200 objects. Until its first thumbnail is ready, the row shows boxes at the bounds of up to 41 of its paths, each in its path's solid fill or stroke color or in gray, when its own paths, without its opacity masks, hold at most 2,000 anchors in all, and an empty white box otherwise. With one path of 200,000 anchors, the first frame after the panel opens takes 2.4 ms instead of 393 ms (release build, median of three runs).
   The same 2,000-anchor limit counts each glyph of type as 4 anchors, each point of a gradient mesh as 1 and every 100 pixels of an image as 1, and a symbol instance or a placed document counts the objects and anchors of the art it shows. A placed document whose file has not been read yet, or a symbol nested more than 8 deep (one that holds an instance of itself, for example), gets its first thumbnail on the thumbnail thread, and so does a row whose art takes more than 20,000 steps to count (each object, subpath, text run and symbol looked at is one step). The first frame after the panel opens takes 2.4 ms instead of 19.8 ms with 5,000 glyphs of point type, 1.8 ms instead of 343 ms with one instance of a symbol of a 200,000-anchor path, 2.0 ms instead of 11.0 ms with a 100 × 100 mesh, and 2.1 ms instead of 274 ms with a placed document of a 200,000-anchor path (release build, median of three runs).
+  When the Layers or Links panel shows another document's rows, the thumbnail thread drops the row thumbnails it has not started for the documents not shown, and it drops all of them when the Layers or Links panel is open after the last document closes. Those rows ask for their thumbnails again when their document shows again, and the thumbnails already made are kept. With 150 rows of 20,000-anchor paths waiting for another document, the 8 rows of such paths of the document shown get their thumbnails in 153 ms instead of 3,265 ms (debug test build, median of three runs).
   In the web build, while a pan, zoom or rotation changes the view of a document whose last canvas render took more than 8 ms, the canvas draws the last frame moved with the view, and it renders the art again once the view has been still for 0.2 s.
   An Alt-drag with the Selection, Direct Selection, Group Selection, Rotate, Scale, Reflect or Shear tool copies the selection on its first move, and the moves after it transform those copies; the previews of the Move, Rotate, Scale, Reflect and Shear dialogs with Copy and Preview on copy the selection once in the same way (#1429).
   An Alt-drag of objects in perspective with the Perspective Selection tool, and an Alt-drag of a plane widget with that tool or the Perspective Grid tool, copy the objects on the first move, and the moves after it move those copies (#1429).