From b07aad512ad92ba5fa46bdc2f49b91dbb2a975ff Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Sun, 1 Nov 2026 05:45:15 +0100
Subject: [PATCH] fix: move the import progress bar message by message for mbox
 files

---
 lib/mail-import/__tests__/engine.test.ts | 8 ++++++++
 lib/mail-import/sources.ts               | 6 +++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/lib/mail-import/__tests__/engine.test.ts b/lib/mail-import/__tests__/engine.test.ts
index 4fb5d6a74..6a481208c 100644
--- a/lib/mail-import/__tests__/engine.test.ts
+++ b/lib/mail-import/__tests__/engine.test.ts
@@ -129,6 +129,14 @@ describe('runImport', () => {
     expect(fake.imported[0].receivedAt).toBe('2020-05-05T10:00:00Z');
   });
 
+  it('moves the progress message by message through a small mbox', async () => {
+    const fake = fakeClient();
+    const fractions: number[] = [];
+    await runImport(options(fake.client, [file(mbox(10))], { onProgress: (p) => { if (p.phase === 'running') fractions.push(p.fraction); } }));
+    const between = new Set(fractions.filter((f) => f > 0 && f < 1).map((f) => f.toFixed(2)));
+    expect(between.size).toBeGreaterThanOrEqual(5);
+  });
+
   it('puts several messages in one Email/import and keeps uploads within the server limit', async () => {
     const fake = fakeClient({ uploadConcurrency: 4 });
     await runImport(options(fake.client, [file(mbox(45))]));
diff --git a/lib/mail-import/sources.ts b/lib/mail-import/sources.ts
index 0d44bae7f..06c66121a 100644
--- a/lib/mail-import/sources.ts
+++ b/lib/mail-import/sources.ts
@@ -180,11 +180,14 @@ async function* mboxFromChunks(
   options: SourceOptions,
   startIndex: number,
   onIndex: (next: number) => void,
+  /** Called with each message's offset in the stream, for a progress that moves message by message. */
+  onOffset?: (offset: number) => void,
 ): ItemSource {
   const splitter = new MboxSplitter({ maxMessageBytes: options.maxMessageBytes });
   let index = startIndex;
   const emit = function* (messages: MboxMessage[]): Generator<ImportItem> {
     for (const m of messages) {
+      onOffset?.(m.offset);
       const i = index++;
       yield i < options.skipBefore ? { kind: "skipped", index: i, reason: "resumed" } : mboxItem(m, i, folder);
     }
@@ -206,7 +209,8 @@ async function* fileChunks(file: Blob, onProgress?: (fraction: number) => void):
 }
 
 export async function* mboxSource(file: File, options: SourceOptions): ItemSource {
-  yield* mboxFromChunks(fileChunks(file, options.onProgress), [], options, 0, () => {});
+  // Reads run ahead in 1 MB pieces; the messages handed over say how far the import really is.
+  yield* mboxFromChunks(fileChunks(file), [], options, 0, () => {}, (offset) => options.onProgress?.(offset / Math.max(1, file.size)));
   options.onProgress?.(1);
 }