- Mount xterm as an absolute inset-0 fill so it always has an explicit box and
fit() measures the real size when the dock expands (was collapsing via the
flex/percentage height chain).
- Acquire the pty slave as the child's controlling terminal (setsid + TIOCSCTTY)
so the kernel delivers SIGWINCH on a browser resize. Previously the initial
size was read once but live resizes were dropped, so the pane stayed at its
startup size — content never grew with the window.
