fix(pi-tui): repaint viewport in place when content collapses above it (#1315)

* fix(pi-tui): repaint viewport in place when content collapses above it

When content shrinks past the viewport top while a line above the
viewport also changes, the clamped differential path left the render
loop empty, cleared deleted lines past the screen bottom, and desynced
the cursor anchor — leaving the viewport blank with the input box gone
until a full redraw.

Repaint the visible viewport in place for that case (no ESC[3J, so the
scrollback and the user's scroll position are preserved), and clamp
deleted-line clearing to the screen bottom so it can never scroll
untracked.

* fix(kimi-code): clear the screen fully on session reset

The collapse repaint in pi-tui intentionally preserves scrollback, so
/new, /clear, and session switches no longer got a clean screen as a
side effect of the destructive full redraw — the previous session's
text stayed above the welcome banner.

Session resets want a pristine screen, so force a destructive full
render explicitly instead of relying on the renderer's shrink
behavior.

* fix(pi-tui): delete kitty images straddling the viewport top on collapse repaint

A multi-row kitty image can start above prevViewportTop while its
reserved rows are still visible. The collapse repaint's image-delete
range started at prevViewportTop and missed the image line carrying the
id, leaving a stale overlay that also dropped out of
previousKittyImageIds tracking. Widen the range to include such a
straddling block.

* chore: shorten session reset changeset wording

* fix(pi-tui): re-anchor the viewport whenever content shrinks below the screen bottom

previousViewportTop only ever grows during normal rendering, so after a
shrink the content bottom could hover above the screen bottom, leaving
dead rows that nothing repaints. Upstream masked this by frequently
doing destructive full redraws, which re-anchored as a side effect; the
fork removed those redraws without replacing the re-anchoring.

Generalize the collapse repaint into a re-anchor check at the top of
the differential path: whenever prevViewportTop exceeds
max(0, newLines - height), repaint the visible viewport in place with
the tail of the new content. The input area snaps back to the screen
bottom, scrollback and the user's scroll position stay intact (no
ESC[3J), and the previous collapse branch becomes a defensive
destructive fallback.

Update the three renderer tests that encoded the old behavior
(destructive redraw on shrink, viewport hover after clamped shrink) to
assert the re-anchored behavior instead.

* fix(kimi-code): full repaint on ctrl+o expansion toggle

Expanding tool output shifts content above the viewport; the clamped
differential render paints the shifted content through the screen,
stacking a duplicate copy below the stale one in scrollback on every
toggle. The toggle is a deliberate user action (like /clear), so do a
destructive full render instead: scrollback holds exactly one copy and
the expanded output stays readable by scrolling up.

* chore: simplify user-facing changeset wording

.changeset/cli-blank-screen-after-shrink.md

@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Fix TUI rendering bugs that caused the screen to go blank and the input box to disappear.

.changeset/cli-session-reset-full-clear.md

@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Clear the screen fully when starting a new session via /new, /clear, or a session switch.

.changeset/pi-tui-collapse-repaint.md

@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/pi-tui": patch
+---
+
+Re-anchor the viewport with an in-place repaint whenever content shrinks below the screen bottom, and clamp deleted-line clearing to the screen bottom, so large shrinks no longer blank the screen, desync the cursor, or leave the UI hovering above dead rows.