Fix browser showOpenFilePicker support

[austinywang]

Summary

• add a browser panel regression test that verifies window.showOpenFilePicker is installed in WKWebView pages
• install a shared File System Access bridge from the browser WKWebView configuration path
• back showOpenFilePicker() with WebKit's native file-input picker and lightweight file handle objects so selected files expose getFile()

Fixes #4050.

Testing

• Not run locally per repo policy; CI will run on the PR.

---

Note

Medium Risk
Injects new JavaScript into every main-frame page load and changes file-picker behavior in the embedded browser, which could impact site compatibility or event-handling edge cases despite being scoped to a shim.

Overview
Restores window.showOpenFilePicker() support in the browser WKWebView by injecting a new BrowserFileSystemAccessBridge user script at document start (main frame only).

The bridge implements a minimal File System Access API shim backed by a hidden <input type="file">, returning lightweight FileSystemFileHandle-like objects with getFile() and ensuring the picker promise settles once (including an AbortError dismissal path with focus-return fallback) while cleaning up DOM inputs.

Adds regression tests to verify the API is installed in loaded pages and that the picker rejects/cleans up correctly when focus returns without a native cancel event.

^{Reviewed by Cursor Bugbot for commit 82f05ca. Bugbot is set up for automated code reviews on this repo. Configure here.}

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Summary by cubic

Restore window.showOpenFilePicker in WKWebView by injecting a minimal File System Access bridge that uses the native file input picker. Ensures the promise settles once, avoids element-focus false cancels, and cleans up; fixes #4050.

• Bug Fixes
  • Inject BrowserFileSystemAccessBridge via WKUserScript at document start (main frame) so showOpenFilePicker() is available on all pages.
  • Shim supports multiple and types.accept, returns lightweight FileSystemFileHandle with getFile(), and uses a window-focus fallback to resolve or reject with AbortError on dismissal; ignores element-focus events.
  • Add tests for API install, window-focus dismissal with AbortError, no rejection on element focus, and cleanup of hidden inputs.

^{Written for commit 82f05ca. Summary will update on new commits.}

[vercel]

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Project	Deployment	Actions	Updated (UTC)
cmux	Canceled		May 15, 2026 3:19am
cmux-staging	Building	Preview, Comment	May 15, 2026 3:19am

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📥 Commits

Reviewing files that changed from the base of the PR and between c83f585 and 82f05ca.

📒 Files selected for processing (2)

• Sources/Panels/BrowserPanel.swift
• cmuxTests/BrowserPanelTests.swift

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-4050-showopenfilepicker-undefined

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[greptile-apps]

Greptile Summary

This PR restores window.showOpenFilePicker() inside the browser WKWebView panel by injecting BrowserFileSystemAccessBridge as a document-start WKUserScript (main frame only). The shim wraps a hidden <input type="file"> and provides a robust single-settle promise with three settlement paths: change (file selected), cancel (macOS 14 native dismiss), and a window.focus-return fallback with a setTimeout(0) deferral so a selection-triggered change always wins.

• BrowserPanel.swift: Adds BrowserFileSystemAccessBridge enum (222-line JS string) and wires it into configureWebViewConfiguration alongside the existing telemetry script injection.
• BrowserPanelTests.swift: Adds BrowserPanelFileSystemAccessBridgeTests with three tests: API presence, focus-return rejection (using callAsyncJavaScript), and a guard confirming internal element focus does not prematurely reject the picker promise.

Confidence Score: 5/5

Safe to merge. The shim is narrowly scoped to main-frame pages, guards against double-installation, and the three settlement paths are mutually exclusive thanks to the settled flag.

The JavaScript bridge is logically sound, the single-settle invariant holds across all three settlement paths (change / cancel / focus-return), and the new tests exercise the non-obvious dismissal flows correctly using callAsyncJavaScript.

No files require special attention. BrowserPanel.swift continues to grow, but the addition here is self-contained.

Important Files Changed

Filename	Overview
Sources/Panels/BrowserPanel.swift	Adds BrowserFileSystemAccessBridge enum (222 lines of JS string + Swift wrapper) and injects it as a document-start WKUserScript. Logic is correct; the only concern is adding to an already 11 k-line file.
cmuxTests/BrowserPanelTests.swift	Adds BrowserPanelFileSystemAccessBridgeTests with three coverage scenarios: API installation, focus-return rejection, and no-reject-on-element-focus. Uses callAsyncJavaScript correctly for promise-settling assertions.

Sequence Diagram

sequenceDiagram
    participant Page as "Web Page JS"
    participant Bridge as "BrowserFileSystemAccessBridge"
    participant Input as "hidden input[type=file]"
    participant OS as "OS File Picker"
    participant Win as "window focus"

    Page->>Bridge: "showOpenFilePicker(options)"
    Bridge->>Input: "createElement + configure"
    Bridge->>Input: "addEventListener(change, cancel)"
    Bridge->>Win: "addEventListener(focus)"
    Bridge->>Input: "appendChild + click()"
    Input->>OS: "open native picker"

    alt "User selects file(s)"
        OS-->>Input: "change event"
        Input->>Bridge: "handleChange()"
        Bridge-->>Page: "resolve [FileSystemFileHandle...]"
    else "User dismisses (cancel event)"
        OS-->>Input: "cancel event"
        Input->>Bridge: "handleCancel()"
        Bridge-->>Page: "reject AbortError"
    else "Window regains focus (fallback)"
        OS-->>Win: "focus event"
        Win->>Bridge: "handleWindowFocus()"
        Bridge->>Bridge: "setTimeout(0) — defer one turn"
        alt "change already settled"
            Bridge->>Bridge: "settled=true, no-op"
        else "no files — true dismissal"
            Bridge-->>Page: "reject AbortError"
        end
    end

    Bridge->>Input: "cleanup: remove listeners + DOM node"

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_{Reviews (4): Last reviewed commit: "fix: avoid element-focus file picker dis..." | Re-trigger Greptile}

[austinywang]

Greptile note checked: cmux targets macOS 14.0 or later (MACOSX_DEPLOYMENT_TARGET = 14.0, and public requirements say macOS 14+). The <input type="file"> cancel event concern is for Monterey and earlier, which this app does not support, so no fallback path is needed here.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[austinywang]

Greptile's latest pass reports this is safe to merge and that no files require special attention. I verified the remaining note is housekeeping around file placement; I'm leaving the bridge in BrowserPanel for this PR so the functional fix and regression coverage stay stable, and the extraction can be handled separately if we want that cleanup.