fix(download): keep the download iframe alive after a successful export

docs/architecture.md

@@ -204,7 +204,10 @@ left waiting for a `controllerchange` that is never coming.
 returns a `WritableStream` backed by the MessagePort sink (one `PULL` credit
 per `WRITE`, so the service worker's 4-chunk queue is the backpressure
 boundary). Aborting the writable errors the response so the browser shows a
-failed download instead of a truncated file.
+failed download instead of a truncated file. The iframe is only removed on
+failure, or when the next download starts: iOS Safari finalises a download
+when its initiating frame goes away, even while accepted bytes are still
+being flushed to disk.
 
 ### `export` (browser only, covered by e2e)
 

src/lib/download/README.md

@@ -25,6 +25,12 @@ activation from the click is still live. The worker cannot see a navigation
 that never reached it, so the page waits for its `downloadStarted` message
 before handing over the `MessagePort` and returning the writable.
 
+The iframe outlives the writable. The worker's `PULL`s only mean the
+browser's network layer has read the bytes, not that they are on disk, and
+iOS Safari finalises a download the moment its initiating frame is removed,
+saving only what had been flushed. So `cleanup()` is for failures; after
+success the iframe stays until the next `startDownload` replaces it.
+
 ## Backpressure and errors
 
 The sink posts one `WRITE` per chunk and waits for the worker's `PULL` before

src/lib/download/index.ts

@@ -24,10 +24,15 @@ export interface DownloadOptions {
 
 export interface Download {
   writable: WritableStream<Uint8Array>;
-  /** Removes the iframe that started the download. */
+  /** Removes the iframe that started the download. Call it only after a
+   *  failure: iOS Safari finalises a download when its initiating frame goes
+   *  away, even while bytes it has already accepted are still being flushed
+   *  to disk, so after success the iframe stays until the next download. */
   cleanup(): void;
 }
 
+let activeIframe: HTMLIFrameElement | undefined;
+
 export function registerDownloadWorker(): void {
   if (!("serviceWorker" in navigator)) return;
   // `updateViaCache: 'none'` bypasses the browser's 24h HTTP-cache rule for sw.js.
@@ -143,11 +148,16 @@ async function prepareSwDownload(
   // Navigate first and synchronously, before any await: Safari only starts a
   // download while the click's user activation is live. The service worker
   // holds the request open until the stream below reaches it.
+  activeIframe?.remove();
   const iframe = document.createElement("iframe");
   iframe.hidden = true;
   iframe.src = url;
   document.body.appendChild(iframe);
-  const cleanup = () => iframe.remove();
+  activeIframe = iframe;
+  const cleanup = () => {
+    iframe.remove();
+    if (activeIframe === iframe) activeIframe = undefined;
+  };
 
   // Nothing is generated until the request is known to have arrived.
   if (!(await waitForDownloadStart(url, DOWNLOAD_START_TIMEOUT_MS))) {

src/lib/export/README.md

@@ -51,5 +51,7 @@ last band is rendered rather than as the last byte is written.
 
 Aborting `signal` stops the mosaic and aborts the download, so the browser
 reports a failed download instead of leaving a truncated PNG. Any other
-failure does the same. The renderer is destroyed and the download's iframe
-removed in `finally`, whatever happened.
+failure does the same, and removes the download's iframe. After success the
+iframe is left in place: iOS Safari finalises a download when its initiating
+frame is removed, even while accepted bytes are still being flushed to disk.
+The renderer is destroyed in `finally`, whatever happened.

src/lib/export/index.ts

@@ -125,10 +125,10 @@ export async function exportMap(opts: ExportOptions): Promise<ExportResult> {
     // The response only errors if the worker is told; a failure before or
     // outside `pipeTo` would otherwise leave a truncated file on disk.
     await writable.abort(err).catch(() => {});
+    cleanup();
     throw err;
   } finally {
     renderer?.destroy();
-    cleanup();
   }
 
   return { bbox: viewportBbox(viewport), zoom: viewport.zoom };