Headless automation launches (launchAgentBackgroundSession) created an
inactive tab via createTab(..., { activate: false }) without first telling
the renderer to background-mount that worktree's terminal surface. As a
result the hidden surface either never mounted (no entry in
mountedWorktreeIdsRef) or mounted with display:none (zero-size, can't be
measured/fit), so the eager PTY buffer never flushed on the first mount —
the run tab showed only the shell prompt until an unmount/remount gave the
off-screen xterm a real layout box.
Dispatch BACKGROUND_MOUNT_TERMINAL_WORKTREE_EVENT immediately before
createTab, mirroring the established renderer-backed Codex startup path in
useIpcEvents. The Terminal.tsx listener adds the worktree to
mountedWorktreeIdsRef and marks it measurable for a 3000ms window, so the
hidden surface renders with opacity-0/pointer-events-none (a measurable box)
instead of display:none, letting the first xterm fit flush the buffer.
Extract the inline measurable-mount block into
background-terminal-worktree-visibility.ts (behavior-identical, now
unit-tested) and add unit + E2E coverage.
Fixes#6244
Co-authored-by: ChaDongWun <66347959+lovewave02@users.noreply.github.com>
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