Right-pane cross-origin downloads silently fail; add agent-drawer trace download(s)

[judell]

Symptom

Any code path in the target app loaded in Bram's right-pane that triggers a browser download — including XMLUI's Inspector Export button — silently no-ops when clicked. No file, no devtools error, no Bram notification.

The same Export button works correctly when the same app is opened in a standalone browser (verified: http://localhost:5173/#/settings → click Inspector icon → Export → xs-trace-*.json lands in ~/Downloads/).

Diagnosis

The right-pane iframe (app/index.html:41-45) and the agent-tools-pane iframe (L52-57) are declared identically — no sandbox, only allow="clipboard-write; clipboard-read". What differs is origin:

iframe	origin	Inspector Export
tools-pane	tauri://localhost (Bram's bundled UI; same-origin to host)	works
right-pane	e.g. http://localhost:5173 (user's project server; cross-origin to host)	silently no-ops

The Export code is the same in both (xmlui-test/xmlui/xs-diff.html:7015-7024 — URL.createObjectURL(blob) + synthetic a.click()). The XMLUI runtime captures events correctly in both (_xsLogs is populated). The iframe attributes are identical. The only differential is the origin.

WKWebView (Tauri's macOS WebView) appears to drop synthetic anchor-click downloads from cross-origin iframes when the host hasn't registered an explicit download handler. I don't have a single authoritative URL pinning the rule down — this is inferred from the symptom plus the same-origin/cross-origin split.

Suggested fix

Register a Tauri WebView download handler on the main Bram window that:

1. Catches downloads from any frame, including the cross-origin right-pane.
2. Writes the file to ~/Downloads/ (or wherever the user has configured).
3. Surfaces a brief notification on success ("Saved to ~/Downloads/<filename>") so the user has confirmation parity with the standalone-browser experience.

This unblocks Inspector Export and any other XMLUI app using Actions.download() (https://docs.xmlui.org/howto/download-a-file-from-an-api) from inside Bram.

Bonus: agent-drawer "Download trace" button(s)

Since Bram already knows about both its own UI and the (potentially XMLUI) target app, it could expose convenience buttons in the agent-tools drawer that skip the Inspector dialog entirely.

(a) Bram's own trace — trivial, same-origin

Bram's UI lives at tauri://localhost, so window._xsLogs is directly accessible from the host context. A button labeled "Download Bram trace" serializes that array and writes to ~/Downloads/bram-xs-trace-<ts>.json. No iframe handshake needed.

(b) Target app's trace — needs cooperation

The right-pane is cross-origin, so Bram can't read its window._xsLogs directly. Two approaches:

• postMessage handshake (preferred): Bram posts {type: "xmlui:requestTrace"} to the right-pane window. A small bootstrap that XMLUI ships (or that Bram auto-injects into the right-pane iframe) responds with {type: "xmlui:trace", logs: [...]}. Bram writes the response to ~/Downloads/app-xs-trace-<ts>.json. The button is enabled only after a positive handshake (i.e., when the target app is XMLUI and Inspector tracing is on).

• Inspector iframe forwarding: When the user has the Inspector dialog open in the target app, its xs-diff.html iframe could postMessage _xsLogs upward on demand. Less general — requires the dialog to be open.

The postMessage handshake is cleaner: a small XMLUI-shipped script could expose _xsLogs (and any future trace channels) via a stable message contract, making any XMLUI app usable as a target.

Repro

1. Run any XMLUI app via python3 -m http.server 5173 on a Bram project (.bram.json server.command: "python3 -m http.server 5173", server.cwd pointing at the XMLUI app dir).
2. Launch Bram.
3. In the right pane, click the Inspector magnifying-glass icon → Inspector dialog opens.
4. Click "Export" — note nothing happens; no devtools error.
5. In a standalone browser, visit the same URL, repeat steps 3-4 — xs-trace-<timestamp>.json appears in ~/Downloads/.

Notes

• The bug section (top) is the actionable item.
• The bonus section (agent-drawer button) is independent and could ship later as a separate enhancement once (a) is fixed — but (b) would also work standalone if you prefer to do (b) before tackling the general download-handler in (a).