The PDF viewer's "save with changes" feature uses
`window.showSaveFilePicker()`, but the PDF extension runs in a
cross-origin iframe (chrome-extension:// inside the app's origin).
Chromium's File System Access API blocks cross-origin subframes from
showing file pickers unless the embedder explicitly allows them via
`ContentClient::IsFilePickerAllowedForCrossOriginSubframe()`.
Chrome overrides this in `ChromeContentClient` to allowlist the PDF
extension origin, but Electron never did — so the picker was always
blocked with a SecurityError.
This adds the same override to `ElectronContentClient`, allowing the
built-in PDF extension origin to bypass the cross-origin check.
Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>
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