Export for Screens: files are encoded and written on the background thread

Export for Screens and the Asset Export panel's Export encoded and wrote
every file on the UI thread, so the window stopped responding until the
last file was written. For 50 artboards in PNG at 1x, 2x and 3x, PDF and
SVG (250 files, 47 MiB), the dialog's OK took a median of 43.8 s in a
release build on Windows.

With Enable Background Export on (Preferences › File Handling), OK now
checks the settings, remembers them and takes a snapshot of the
document, and the dialog closes. The thread that writes background
saves and exports then encodes the files and writes them into the
folder, or one ZIP in it, and the export progress dialog's Cancel stops
it before the next file. The same OK takes under 0.001 s, and the files
have the same names and bytes. With the preference off, without a
folder, on the web and over MCP and scripts (document.exportForScreens
and assets.export), the files are made at once with the same reply as
before.

Both commands report one progress step per file, so the progress dialog
shows "File 3 of 250" and MCP progress notifications count the files in
place of the encoders' artboards and pages.

Docs: docs/target-app-parity.md, docs/control-protocol.md, docs/mcp.md.

docs/control-protocol.md

@@ -118,7 +118,7 @@ the two can still win that race.
 | `app.export` Text and WebP | `{path?, format: txt, encoding?: utf8 \| utf16, lineEndings?: lf \| crlf, selectionOnly?}` / `{format: webp, lossless?, quality?}` | Text writes the stories in stacking order (back to front; a thread once; hidden objects and template layers left out), each line ended by `lineEndings`; `utf16` is little-endian with a byte order mark. Export As → Text opens `txtOptions` (fields `path`, `encoding`, `lineEndings`, `selectionOnly`; OK writes the file). WebP is always lossless: `lossless: false` adds a warning |
 | `engine.execute` `document.exportForOffice` | `{}` / `{path?, ppi?: 72 \| 150 \| 300, transparent?, artboard?}` | File → Save for Office Documents…: without params opens `saveForOffice` (fields `ppi`, `transparent`, `artboard`; OK picks the file); with params writes the artboard as a PNG (`path`, else a picked file) → `{path}`, and the status bar shows the first of the export's `warnings` as Save As shows a save's notes. Headless it returns `{dataBase64, width, height, warnings}` |
 | Native file dialogs | any command with no `path` that needs one (`file.open`, `file.saveAs`, `file.place`, exports…) | on macOS (a sheet on the window) and Linux the dialog doesn't hold the window: the call returns `"cancelled"` at once, `ui.inspect` reports `fileDialog`, and when the person at the computer picks a file the command runs again on the document it was asked for. While it is open another dialog is refused: that call returns `"cancelled"` too, and the status bar (`ui.inspect` → `ui.status`) says `"A file dialog is open: choose there first"`; on macOS `app.quit`, `file.close` and `file.closeAll` return that message and the menu bar does nothing (#592, #867). On Windows the dialog is shown in line: the call returns once it closes. Pass `path` to skip the dialog |
-| Background Save / Export | `app.save`, `app.export {path}`, `file.save`, `file.saveAs`, `file.saveCopy` | with the `backgroundSave` / `backgroundExport` preferences on (the default) the desktop app writes the file from a snapshot on a worker thread: the call returns `{path, background: true, status}` at once, `ui.inspect` lists the jobs still running in `background` (labels such as `"Saving a.vectorcraft"`), and the document counts as saved (as of the snapshot) once its file is written. Quit and Close wait for them. With the preferences off (`prefs.set`), saves return once written |
+| Background Save / Export | `app.save`, `app.export {path}`, `file.save`, `file.saveAs`, `file.saveCopy`, `file.exportForScreens {folder, …}` | with the `backgroundSave` / `backgroundExport` preferences on (the default) the desktop app writes the file from a snapshot on a worker thread: the call returns `{path, background: true, status}` at once (`file.exportForScreens`: `{background: true, status}`, and the files are written into the folder from a snapshot taken at the call), `ui.inspect` lists the jobs still running in `background` (labels such as `"Saving a.vectorcraft"`), and the document counts as saved (as of the snapshot) once its file is written. Quit and Close wait for them. With the preferences off (`prefs.set`), saves return once written |
 | `engine.execute` `file.exportForScreens` | `{}` / `{…document.exportForScreens params}` | without params opens `exportForScreens` on the settings the document last exported with (`document.exportSettings`). While the Artboard tool is in use, the artboards it has selected (its `selected` option, the rows the Artboards panel highlights) are checked instead, and `select` is `range`, or `all` when every artboard is selected. Fields: `tab` (`artboards` \| `assets` \| `layers` \| `selection`; OK reads Export Layer or Export Selection on those), `layers` ([layer id…] checked), `selectionMode` (`one` \| `each`), `name` (the file name pattern of `document.exportForScreens`), `counterStart`, `folders` (`none` \| `rows` \| `format` \| `item` \| `itemFormat`), `exportPreset` (the preset applied last), `__savePreset` (the name typed to save one), `select` (`all` \| `range` \| `full`), `range` ("1-3, 5"), `boards` (a bool per artboard; checking them sets `select` and `range`), `includeBleed`, `folder` (desktop; default the Desktop, else home), `openLocation`, `subfolders`, `prefix`, `preset` (`""` \| `mobile` \| `density`), `formats` (rows `{format, scale: "2x" \| "100w" \| "100h" \| "72ppi", suffix, quality?, folder?}`), `settings` (`{png \| png8 \| jpg \| webp \| gif \| svg \| pdf: {…options}}`, the gear's Format Settings; `__settings` names the format shown). `ui.dialog.confirm` exports (a bad range or no folder keeps it open); on the web OK reads "Download" and the files download (several as one `.zip`). With params it runs `document.exportForScreens`: no `folder` returns the files (or with `zip: true` one store-only zip) as `dataBase64`; with `openLocation` the first file is shown in the file manager. When `warnings` is not empty, an export into a folder or a download shows the first in the status bar as Save As shows a save's notes |
 | Export for Screens' Assets tab, Asset Export panel | `ui.dialog.set` `tab: "assets"`, `assets: [asset id…]`; `window.panel {panel: "assetExport"}` | on the Assets tab of `exportForScreens` the `assets` field lists the checked asset ids (all of them when it opens); `ui.dialog.confirm` runs `document.exportForScreens {assets}` (nothing checked keeps it open) and OK reads "Export Asset". The Asset Export panel acts through `assets.*` and shares the format rows with the dialog (`assets.settings.set`) |
 | `engine.execute` `file.exportSelection` | `{multiple?}` | File › Export Selection…: collects the selected objects as one asset (`assets.add`; one per object with `multiple: true`) and opens `exportForScreens` on its Assets tab with just them checked → `{assets}`. `document.exportSelection` still exports the selection in one go, without the dialog |

docs/mcp.md

@@ -271,7 +271,8 @@ while an export runs (#2165, #2171, #2172):
 - **Progress.** A request with `_meta.progressToken` gets `notifications/progress`
   ([spec](https://modelcontextprotocol.io/specification/2025-06-18/basic/utilities/progress)) before
   its reply: the PDF writer reports "Writing page 2 of 5" with `progress` 2 and `total` 5, raster
-  export reports each artboard, and the PDF flattening stage reports a message without a total. `progress` only grows, updates come at most every 200 ms (the last page always),
+  export reports each artboard, `document.exportForScreens` and `assets.export` report each file
+  ("Exporting file 3 of 250", in place of their encoders' steps), and the PDF flattening stage reports a message without a total. `progress` only grows, updates come at most every 200 ms (the last page always),
   and none follows the reply. A client that does not read fast enough misses updates, never the reply.
 - **A busy engine.** `ping`, `tools/list`, `resources/list`, `resources/templates/list`,
   `prompts/list` and `prompts/get` need no engine and are answered at once, whatever is running.

docs/target-app-parity.md

@@ -908,6 +908,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - On the desktop, File › Place reads and decodes files off the UI thread, one at a time, and places each once it is read, with "Placing" in the status bar and the busy spinner until then: the Place dialog's OK, a file dropped on the canvas, a click or drag of the place cursor, and the OK of the PDF, DXF and text import options. The files and thumbnails the place cursor is loaded with are read the same way, and the Place dialog's lines about its files are read on a thread of their own. Files are placed in the order they were started; a file is not placed while a dialog is open or a drag is under way, and a file whose document was closed meanwhile is not placed. `file.place` and `file.place.queue` over MCP and scripts reply as before.
   - On the desktop, Copy and Cut make the system clipboard's formats (text, SVG, PDF, PNG) from a copy of the clipboard on the thread that writes background saves and exports, and write them when they are ready; while that thread has a save, an export, a print or a recovery copy to finish, they are made and written at once. Until they are written, Paste in VectorCraft takes the internal clipboard, and while a text field or the Type tool has the keyboard the app waits for them. Another Copy or Cut, or a copy of text in VectorCraft, drops them.
   - On the desktop with Enable Background Export on (Preferences › File Handling), File › Package… collects the linked files and fonts and writes the package from the document as it was at OK on the thread that writes background saves and exports, with "Packaging" in the status bar and the busy spinner until it is written; a save of unsaved changes that Package starts is written first. `file.package` over MCP and scripts replies as before.
+  - On the desktop with Enable Background Export on, Export for Screens and the Asset Export panel's Export encode and write their files (into the folder, or one ZIP in it) from the document as it was at OK on the thread that writes background saves and exports, with "Exporting" in the status bar and the export progress dialog, which counts the files and whose Cancel stops before the next file. `document.exportForScreens` and `assets.export` over MCP and scripts reply as before.
   - Missing linked files can be looked for in a folder and its subfolders: Find in Folder… in the Linked File Not Found dialog and in the Links panel menu lists the files with each one's name, those with the size and contents saved with the link first, and relinks the files chosen, copying an image's file into the document's folder when asked, without replacing a file there (`links.findFiles`, `links.relink {copy}`). In the Links panel it looks for the selected linked files when any are selected.
   - On Windows, a document opened from a mapped network drive finds its links stored with the share's own path (`\\server\share\…`), as the same document opened from the share does; links on other computers stay missing.
   - On Windows, a link on a drive mapped to a network share other than the document's (`Y:\…`) is missing, with the reason that the path is on another computer, as a link stored with that share's own path is; links on this computer's drives and on the document's share are followed.