Codex (Ratatui) emits an 8x RI burst on startup right after setting
DECSTBM. RI at the top of the scroll region scrolls the region down,
and DECSTBM only constrains rows -- so the scroll spans every column
and drags the right-rail session-tree entries down with the main pane.
The chrome cache then hid the clobber because the computed sidebar
frame was unchanged.
The viewport renderer now flags any chunk containing RI / IND / NEL /
SU / SD / IL / DL and OnPTYOut drops the sidebar cache when the flag
is set, so the next drawSidebar repaints over the drift.
Adds unit tests for the new flag and a harness regression scenario
(sidebar_survives_ri_scroll) that fails without the fix.
Harry Bayliss
58dbb56937