quota: Fix maildir quota drop to zero after IMAP MOVE

src/plugins/quota/quota-private.h

@@ -165,6 +165,11 @@ struct quota_transaction_context {
 	bool auto_updating:1;
 	/* Quota doesn't need to be updated within this transaction. */
 	bool no_quota_updates:1;
+	/* TRUE if this transaction is for an IMAP MOVE operation. Used by
+	   quota_try_alloc() to avoid double-counting the expunged source
+	   mail: quota_mail_expunge() already accounts for it in the source
+	   transaction via quota_free_bytes(). */
+	bool moving:1;
 };
 
 void quota_add_user_namespace(struct quota *quota, const char *root_name,

src/plugins/quota/quota-storage.c

@@ -255,6 +255,7 @@ static int quota_check(struct mail_save_context *ctx)
 	if (qt->failed)
 		return 0;
 
+	qt->moving = ctx->moving;
 	const char *error;
 	ret = quota_try_alloc(qt, ctx->dest_mail, ctx->expunged_mail,
 			      NULL, &error);

src/plugins/quota/quota.c

@@ -1180,7 +1180,14 @@ quota_try_alloc(struct quota_transaction_context *ctx,
 	   quota_alloc() or quota_free_bytes() was already used within the same
 	   transaction, but that doesn't normally happen. */
 	ctx->auto_updating = FALSE;
-	quota_alloc_with_size(ctx, size, expunged_size);
+	/* For IMAP MOVE, do not subtract expunged_size here: the source mail
+	   expunge is already accounted for by quota_mail_expunge() ->
+	   quota_free_bytes() in the source transaction.  Subtracting it here
+	   too causes double-counting (destination records bytes_used=0, source
+	   records bytes_used=-size, net result: quota drops by size).
+	   expunged_size is still passed to quota_test_alloc() above so that
+	   moves at the quota boundary are correctly permitted. */
+	quota_alloc_with_size(ctx, size, ctx->moving ? 0 : expunged_size);
 	return QUOTA_ALLOC_RESULT_OK;
 }