Add a side-card preference 'browserAllowedLoopback' (comma-separated
host or host:port entries) that lets the sidebar browser navigate to
explicitly trusted loopback addresses. Bare hosts (localhost,
127.0.0.1) match every port; host:port entries match exactly.
Allowlisted local addresses additionally get the 'allow-same-origin'
sandbox token in the iframe: local dev servers (Vite etc.) need a real
origin for their module/HMR/fetch pipeline and would otherwise render
blank in the opaque-origin sandbox. The page stays cross-origin to the
GUI and to every other site.
The server-side browser.probe route mirrors the same allowlist so
probed local servers are not rejected as 'local addresses are not
probed'.
trinhthiminh650-tech