Never paint the fleet board past the rows it was granted

The formatter sizes the board to its content, but collapse can grant it
fewer rows than that. The rows were rendered before the resolver ran and
were never clamped to its answer, so a dozen lanes on an 80x24 terminal
added thirteen children to a seven-row box: rows painted on top of each
other and on the transcript beneath, and the churn tore down text buffers
the next repaint then wrote to.

Rendering now happens after the resolver has spoken and only ever paints
what it granted. Lanes that no longer fit are disclosed the same way the
formatter discloses them, so the count stays honest all the way down.

Author: TheGreatAxios

src/tui-opentui/chrome-state.ts

@@ -269,6 +269,46 @@ export function formatAgentsPanel(
   return rows
 }
 
+/**
+ * Fit the board into the rows geometry actually granted it.
+ *
+ * The formatter sizes the board to its content, but collapse can grant fewer
+ * rows than that under pressure. Painting the full set anyway overflows the
+ * zone's box — rows land on top of each other and on whatever is below. So the
+ * granted height is the last word, and the lanes it costs are disclosed rather
+ * than dropped in silence.
+ */
+export function clampBoardRows(
+  rows: readonly AgentPanelRow[],
+  height: number,
+): readonly AgentPanelRow[] {
+  if (height <= 0) return []
+  if (rows.length <= height) return rows
+
+  const header = rows[0]
+  if (header === undefined) return []
+  const lanes = rows.filter((r) => r.kind === "lane")
+
+  // Below a few rows the disclosure line costs more than the lane it displaces,
+  // so the header carries the count instead — the same trade the formatter makes.
+  if (height < 4) {
+    const shown = lanes.slice(0, height - 1)
+    return [withHiddenCount(header, lanes.length - shown.length), ...shown]
+  }
+
+  const shown = lanes.slice(0, height - 2)
+  const hidden = lanes.length - shown.length
+  return [
+    header,
+    ...shown,
+    { label: `+${hidden} more lanes`, tail: "", stalled: false, kind: "more" },
+  ]
+}
+
+function withHiddenCount(header: AgentPanelRow, hidden: number): AgentPanelRow {
+  return hidden > 0 ? { ...header, tail: ` · +${hidden} hidden` } : header
+}
+
 /**
  * The one-line answer to "is everything fine". Counts run worst-first so that
  * a narrow terminal ellipsizes away the routine tail rather than the trouble.

src/tui-opentui/shell.ts

@@ -6,7 +6,7 @@
  */
 
 import { homedir } from "node:os"
-import type { AgentPanelRow, TaskPanelRow } from "./chrome-state.js"
+import { clampBoardRows, type AgentPanelRow, type TaskPanelRow } from "./chrome-state.js"
 
 import {
   BoxRenderable,
@@ -4269,32 +4269,36 @@ export function setChromeZones(
   if (taskChanged) {
     renderTasksRows(shell, bag.chrome.task, shell.layout.contentWidth)
   }
-  if (agentsChanged) {
-    renderAgentsRows(shell, bag.chrome.agents, shell.layout.contentWidth)
-  }
-
   // Only a zone appearing/disappearing or its row count changing alters the
   // row budget; retitling a zone whose row count is unchanged must not
   // re-resolve and re-apply the whole layout.
-  if (
-    taskRowCount === bag.visibility.task &&
-    agentsRowCount === bag.visibility.agents
-  ) {
-    paintChrome(shell)
-    return
+  const budgetUnchanged =
+    taskRowCount === bag.visibility.task && agentsRowCount === bag.visibility.agents
+  if (!budgetUnchanged) {
+    relayout(shell, {
+      visibility: {
+        ...bag.visibility,
+        task: taskRowCount,
+        agents: agentsRowCount,
+      },
+      overlayMode: bag.overlayMode,
+      ...(bag.overlayBodyRows !== undefined
+        ? { overlayBodyRows: bag.overlayBodyRows }
+        : {}),
+    })
   }
 
-  relayout(shell, {
-    visibility: {
-      ...bag.visibility,
-      task: taskRowCount,
-      agents: agentsRowCount,
-    },
-    overlayMode: bag.overlayMode,
-    ...(bag.overlayBodyRows !== undefined
-      ? { overlayBodyRows: bag.overlayBodyRows }
-      : {}),
-  })
+  // Painted after the resolver has spoken, and only ever as many rows as it
+  // granted: a board that paints past its box lands on top of the transcript
+  // and tears down the renderables underneath it.
+  if (agentsChanged || !budgetUnchanged) {
+    renderAgentsRows(
+      shell,
+      clampBoardRows(bag.chrome.agents, shell.layout.heights.agents),
+      shell.layout.contentWidth,
+    )
+  }
+  if (budgetUnchanged) paintChrome(shell)
 }
 
 /** How long a panel-visibility flash holds the notice row. */