[Forge Custom UI] OS file drag-and-drop blocked inside sandboxed iframe when Jira issue is opened as popup/modal (board view)

Forge
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We are building a Forge Custom UI app (jira:issuePanel) that allows users to drag and drop files from their operating system file manager into a file upload panel. The feature works correctly when a Jira issue is opened via its direct URL (full-page view), but completely fails when the issue is opened as a popup modal (the default behaviour when clicking an issue card from a board, backlog, or sprint view).

Steps to reproduce:

  1. Install a Forge Custom UI app with jira:issuePanel that contains any HTML5 drag-and-drop file upload zone.

  2. Navigate to a Jira board, backlog, or sprint view.

  3. Click on any issue card — Jira opens the issue as a popup/floating dialog.

  4. Attempt to drag a file from the OS file manager and drop it anywhere inside the Forge panel.

  5. Expected: The drop zone is highlight and receives the file.

  6. Actual: The drag cursor shows nothing. No dragenter, dragover, or drop events are fired inside the Forge iframe.