Web: a pan or zoom of a heavy document moves the last frame and renders once the view is still

The web build has no render worker, so every frame of a pan, zoom or
rotation of the view rendered the whole canvas on the browser's main
thread before it was shown. In Chrome, each of the 20 steps of a
Space-drag over a document of 50,000 paths took a frame of 533 to
900 ms.

In the web build the canvas now waits while only the view changes on
a document whose last render took more than 8 ms. It draws the last
frame moved, scaled or turned with the view, as the desktop app does
while its worker renders, and it renders once the view has been still
for 0.2 s. The same Space-drag now keeps every frame within 17 ms and
renders once, in 667 to 767 ms, after the view stops. After an edit, a
preview or an option change the canvas renders at once. The desktop
app renders as before, with or without its worker.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -822,6 +822,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   A selection with more than 100,000 points to outline shows its bounds instead of each object's outline and anchors. Paths count their anchors. Area type and type on a path count the anchors of their frame or path, and point type counts six: its box's corners and the ends of its baseline marker. Images, symbol instances, placed documents, and area type or type on a path in perspective count the four corners of their box, and gradient meshes and envelopes count their points and the ends of their mesh lines. The hover highlight leaves out an object past that count, and such an object shows its bounds as the key object. With 32,768 type objects, images, symbol instances, gradient meshes or envelopes selected, the selection's outlines and anchors add fewer than 100 vertices to the canvas's mesh (#2460).
   Delete, Cut, Group, Ungroup, Make and Release Compound Path, Make Clipping Mask, Send to Current Layer, Duplicate, Copy in the Move, Rotate, Scale, Reflect and Shear dialogs, Transform Each with Copy and each move of an Alt-drag take time in proportion to the size of the document (#1429).
   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.
+  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).
   When the desktop app opens a document on a thread of its own (#1245), that thread also reads the document's linked files and lays out its type, and the UI thread only adds the finished document: in release builds, the step left on the UI thread takes 0.02 ms instead of 1.04 s for a document with 12 linked 3000 × 2000 TIFFs, and it takes 0.1 ms instead of 0.85 s for one with 300 Auto Size area type objects, every second one in three columns.