Serial/parallel transfers setting

Transfers: Single | Multi in the options popover, with an 'At once' picker
(2/3/4/6/8) when Multi. Engine concurrency becomes an instance field applied
live: at boot from settings.json and on every saveSettings (raising the limit
starts queued jobs immediately). Verified headless: 3 slow jobs overlap at
concurrency 3, stay strictly serial at 1, clamp works.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/bun/index.ts

@@ -1,5 +1,6 @@
 import { BrowserWindow, Updater } from "electrobun/bun";
 import { BRANDING } from "../shared/branding";
+import { effectiveConcurrency } from "../shared/types";
 import { SessionRegistry } from "./fs/registry";
 import { createAppRPC } from "./rpc-handlers";
 import { ConnectionStore } from "./store/connections";
@@ -58,5 +59,6 @@ new BrowserWindow({
 
 engine.onUpdate = (snapshot) => rpc.send.transferUpdate(snapshot);
 events.sessionClosed = (info) => rpc.send.sessionClosed(info);
+void settings.load().then((s) => engine.setConcurrency(effectiveConcurrency(s)));
 
 console.log(`${BRANDING.appName} started (${url})`);

src/bun/rpc-handlers.ts

@@ -1,6 +1,6 @@
 import { BrowserView } from "electrobun/bun";
 import type { AppRPC } from "../shared/rpc";
-import type { Result } from "../shared/types";
+import { effectiveConcurrency, type Result } from "../shared/types";
 import type { SessionRegistry } from "./fs/registry";
 import { SftpProvider } from "./fs/sftp";
 import type { ConnectionStore } from "./store/connections";
@@ -144,6 +144,7 @@ export function createAppRPC({ registry, engine, store, settings, events }: AppD
 				saveSettings: ({ settings: next }) =>
 					wrap(async () => {
 						await settings.save(next);
+						engine.setConcurrency(effectiveConcurrency(next));
 						return {};
 					}),
 			},

src/bun/transfers/engine.ts

@@ -5,8 +5,6 @@ import type { FileSystemProvider } from "../fs/provider";
 import type { SessionRegistry } from "../fs/registry";
 import { walkTree } from "./walk";
 
-// One at a time keeps remote servers happy; bump for parallel jobs.
-const TRANSFER_CONCURRENCY = 1;
 const PROGRESS_INTERVAL_MS = 100;
 
 interface Job {
@@ -41,9 +39,16 @@ export class TransferEngine {
 	private running = 0;
 	private counter = 0;
 	private lastEmit = new Map<string, number>();
+	// serial by default; the transfers setting raises it (jobs are independent)
+	private concurrency = 1;
 
 	constructor(private registry: SessionRegistry) {}
 
+	setConcurrency(n: number): void {
+		this.concurrency = Math.max(1, Math.min(8, Math.round(n) || 1));
+		this.pump(); // a raised limit starts queued jobs immediately
+	}
+
 	enqueue(params: {
 		srcSessionId: string;
 		items: { path: string; type: EntryType }[];
@@ -119,7 +124,7 @@ export class TransferEngine {
 	}
 
 	private pump(): void {
-		while (this.running < TRANSFER_CONCURRENCY && this.queue.length) {
+		while (this.running < this.concurrency && this.queue.length) {
 			const job = this.queue.shift()!;
 			this.running++;
 			this.run(job)

src/shared/types.ts

@@ -46,14 +46,25 @@ export interface AppSettings {
 	speedUnit: "MBps" | "Mbps";
 	/** Font size of the path bar and file list. */
 	textSize: "compact" | "default" | "large";
+	/** Run transfer jobs one at a time, or several in parallel. */
+	transferMode: "single" | "multi";
+	/** How many jobs run at once when transferMode is "multi". */
+	transferConcurrency: number;
 }
 
 export const DEFAULT_SETTINGS: AppSettings = {
 	theme: "system",
 	speedUnit: "MBps",
 	textSize: "default",
+	transferMode: "single",
+	transferConcurrency: 3,
 };
 
+export function effectiveConcurrency(settings: AppSettings): number {
+	if (settings.transferMode === "single") return 1;
+	return Math.max(1, Math.min(8, Math.round(settings.transferConcurrency) || 1));
+}
+
 export type TransferState =
 	| "queued"
 	| "scanning"

ui/src/app/settings.service.ts

@@ -7,6 +7,8 @@ export class SettingsService {
   readonly theme = signal<AppSettings['theme']>(DEFAULT_SETTINGS.theme);
   readonly speedUnit = signal<AppSettings['speedUnit']>(DEFAULT_SETTINGS.speedUnit);
   readonly textSize = signal<AppSettings['textSize']>(DEFAULT_SETTINGS.textSize);
+  readonly transferMode = signal<AppSettings['transferMode']>(DEFAULT_SETTINGS.transferMode);
+  readonly transferConcurrency = signal(DEFAULT_SETTINGS.transferConcurrency);
 
   private readonly rpc = inject(RpcService);
 
@@ -17,6 +19,8 @@ export class SettingsService {
       this.theme.set(res.settings.theme);
       this.speedUnit.set(res.settings.speedUnit);
       this.textSize.set(res.settings.textSize);
+      this.transferMode.set(res.settings.transferMode);
+      this.transferConcurrency.set(res.settings.transferConcurrency);
       this.applyTheme();
       this.applyTextSize();
     });
@@ -39,6 +43,16 @@ export class SettingsService {
     void this.persist();
   }
 
+  setTransferMode(mode: AppSettings['transferMode']): void {
+    this.transferMode.set(mode);
+    void this.persist();
+  }
+
+  setTransferConcurrency(count: number): void {
+    this.transferConcurrency.set(count);
+    void this.persist();
+  }
+
   private applyTheme(): void {
     const theme = this.theme();
     if (theme === 'system') {
@@ -62,6 +76,8 @@ export class SettingsService {
       theme: this.theme(),
       speedUnit: this.speedUnit(),
       textSize: this.textSize(),
+      transferMode: this.transferMode(),
+      transferConcurrency: this.transferConcurrency(),
     };
   }
 

ui/src/app/tab-bar/tab-bar.ts

@@ -10,6 +10,8 @@ const SPEED_UNITS = [
   { value: 'Mbps', label: 'Mbps' },
 ] as const;
 const TEXT_SIZES = ['compact', 'default', 'large'] as const;
+const TRANSFER_MODES = ['single', 'multi'] as const;
+const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
 
 @Component({
   selector: 'app-tab-bar',
@@ -136,6 +138,42 @@ const TEXT_SIZES = ['compact', 'default', 'large'] as const;
               }
             </div>
           </div>
+
+          <div class="mt-2.5 flex items-center justify-between gap-2">
+            <span class="text-xs text-muted-foreground">Transfers</span>
+            <div class="flex rounded-md border border-border p-0.5">
+              @for (mode of transferModes; track mode) {
+                <button
+                  (click)="settings.setTransferMode(mode)"
+                  class="rounded px-2 py-0.5 text-[11px] capitalize"
+                  [class.bg-accent]="settings.transferMode() === mode"
+                  [class.text-foreground]="settings.transferMode() === mode"
+                  [class.text-muted-foreground]="settings.transferMode() !== mode"
+                >
+                  {{ mode }}
+                </button>
+              }
+            </div>
+          </div>
+
+          @if (settings.transferMode() === 'multi') {
+            <div class="mt-2.5 flex items-center justify-between gap-2 pl-3">
+              <span class="text-xs text-muted-foreground">At once</span>
+              <div class="flex rounded-md border border-border p-0.5">
+                @for (count of concurrencyChoices; track count) {
+                  <button
+                    (click)="settings.setTransferConcurrency(count)"
+                    class="rounded px-2 py-0.5 text-[11px] tabular-nums"
+                    [class.bg-accent]="settings.transferConcurrency() === count"
+                    [class.text-foreground]="settings.transferConcurrency() === count"
+                    [class.text-muted-foreground]="settings.transferConcurrency() !== count"
+                  >
+                    {{ count }}
+                  </button>
+                }
+              </div>
+            </div>
+          }
         </div>
       }
     </div>
@@ -149,6 +187,8 @@ export class TabBar {
   readonly themes = THEMES;
   readonly speedUnits = SPEED_UNITS;
   readonly textSizes = TEXT_SIZES;
+  readonly transferModes = TRANSFER_MODES;
+  readonly concurrencyChoices = CONCURRENCY_CHOICES;
 
   close(event: MouseEvent, id: number): void {
     event.stopPropagation();