fix: prevent duplicate window when opening external URL on cold start

[iamEvanYT]

Summary

Fixes a bug where opening a link from another app (while Flow is set as default browser and is closed) creates two windows instead of opening the URL as a tab in the restored session window.

Root Cause

When the app cold-starts via an external URL, two independent async paths race with no coordination in browser.ts:

1. processInitialUrl() (or macOS open-url event) → handleOpenUrl() sees 0 windows → creates Window Custom Page Load Error #1
2. runOnboardingOrInitialWindow() → restoreSession() → creates Window Redesign #2 from persisted tabs

Both run concurrently, neither knows about the other, and both independently create windows.

Fix

• src/main/app/urls.ts: Added a startup URL queue. During cold start (startupComplete === false), handleOpenUrl() queues URLs instead of creating windows immediately. New flushPendingUrls() function sets the flag and opens all queued URLs using the already-existing restored windows.
• src/main/app/onboarding.ts: After session restore completes, calls flushPendingUrls() so queued URLs open as new tabs in the restored window. Also properly awaits createInitialWindow() which was previously fire-and-forget.

Behavior After Fix

Scenario	Before	After
Cold start via external URL (with saved session)	2 windows	1 window (URL opens as tab in restored window)
Cold start via external URL (no saved session)	1 window	1 window (no change)
URL opened while app is already running	1 window	1 window (no change — startupComplete is already true)

[github-actions]

Build artifacts for all platforms are ready! 🚀

Download the artifacts for:

• Linux
• Linux (arm64)
• macOS
• macOS (Intel)
• Windows

One-line installer (Unstable):

bunx flow-debug-build --open 22586327955

(execution 22586327955 / attempt 1)

[greptile-apps]

Greptile Summary

This PR fixes a race condition where opening an external URL during cold start created two windows instead of one. The fix introduces a queueing mechanism: URLs received before startup completes are queued and then flushed after the initial window (either onboarding or session restore) is created, ensuring they open as tabs in the existing window.

Key changes:

• urls.ts: Added pendingStartupUrls queue and startupComplete flag to coordinate URL handling during startup
• onboarding.ts: Calls flushPendingUrls() after window creation and properly awaits createInitialWindow() to ensure synchronization

The solution elegantly coordinates two previously-independent async paths (processInitialUrl()/open-url event and runOnboardingOrInitialWindow()) that were racing to create windows.

Confidence Score: 5/5

• Safe to merge with no issues found
• The implementation is clean, well-documented, and correctly addresses the race condition. The queueing mechanism is thread-safe (JavaScript is single-threaded), error paths are handled appropriately, and the await addition ensures proper sequencing. The code changes are focused and minimal.
• No files require special attention

Important Files Changed

Filename	Overview
src/main/app/urls.ts	Added URL queueing mechanism with pendingStartupUrls and startupComplete flag to prevent race condition during cold start
src/main/app/onboarding.ts	Added flushPendingUrls() calls after window creation and properly awaits createInitialWindow() to ensure window exists before processing queued URLs

Sequence Diagram

sequenceDiagram
    participant CLI as Command Line / OS
    participant URL as urls.ts
    participant OB as onboarding.ts
    participant WC as Window Controller
    
    Note over CLI,WC: Before Fix (Race Condition)
    CLI->>URL: processInitialUrl()
    CLI->>OB: runOnboardingOrInitialWindow()
    par Parallel Execution
        URL->>WC: handleOpenUrl() sees 0 windows
        WC->>WC: Create Window #1 for URL
    and
        OB->>WC: restoreSession()
        WC->>WC: Create Window #2 from session
    end
    Note over CLI,WC: Result: 2 windows created
    
    Note over CLI,WC: After Fix (Coordinated)
    CLI->>URL: processInitialUrl()
    CLI->>OB: runOnboardingOrInitialWindow()
    URL->>URL: handleOpenUrl() checks startupComplete=false
    URL->>URL: Queue URL in pendingStartupUrls
    OB->>WC: await restoreSession()
    WC->>WC: Create Window from session
    OB->>URL: flushPendingUrls()
    URL->>URL: Set startupComplete=true
    URL->>WC: Open queued URLs as tabs in existing window
    Note over CLI,WC: Result: 1 window with URL as tab

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_{Last reviewed commit: a150491}