Cover the slash-command picker's height bounding with tests

Investigation for CL-5412 found the picker already routes through
the shared list-viewport/resolveGeometry machinery: a 50-entry
catalog stays bounded at 24, 16, 12, 8, and 6 terminal rows, the
prompt box below it stays intact, and moving the selection scrolls
the window so the active row is always visible. No sizing logic was
missing, so this adds the regression coverage the ticket asked for
rather than a second windowing path.

Closes CL-5412.

Author: TheGreatAxios

src/tui-opentui/palette-paint.test.ts

@@ -8,9 +8,11 @@ import type { KeyEvent } from "@opentui/core"
 
 import { withTestRenderer } from "./harness"
 import { openModelPickerOverlay } from "./overlays"
+import type { PaletteCommand } from "./palette"
 import {
   createAppShell,
   handlePaletteFilterKey,
+  moveOverlaySelection,
   openPalette,
   type AppShell,
 } from "./shell"
@@ -256,3 +258,63 @@ describe("command palette selection colour", () => {
   })
 })
 
+describe("command palette height cap", () => {
+  const BIG_CATALOG: readonly PaletteCommand[] = Array.from(
+    { length: 50 },
+    (_, i) => ({
+      id: `cmd_${String(i)}`,
+      label: `Fake command number ${String(i)} with a longish label`,
+      dispatch: "command" as const,
+    }),
+  )
+
+  // Every plugin-inflated catalog and every terminal size gets a bounded
+  // frame: the border-to-border row count above the prompt box never grows
+  // past the terminal, and the box below stays intact and readable.
+  for (const height of [24, 16, 12, 8, 6]) {
+    test(`stays within a ${height}-row terminal and keeps the prompt box intact`, async () => {
+      await withTestRenderer(
+        async (h) => {
+          const shell = createAppShell(h.renderer, {
+            terminal: { columns: 80, rows: height },
+            wireKeys: false,
+            run: "idle",
+          })
+          openPalette(shell, { catalog: BIG_CATALOG, title: "commands · /" })
+          await h.renderOnce()
+          const lines = h.captureCharFrame().split("\n")
+          // captureCharFrame's trailing newline yields one extra split
+          // element — the frame itself must not exceed the terminal rows.
+          expect(lines.length).toBeLessThanOrEqual(height + 1)
+          expect(lines.some((l) => l.includes("message…"))).toBe(true)
+        },
+        { width: 80, height },
+      )
+    })
+  }
+
+  test("scrolling the selection keeps the active row inside the window", async () => {
+    await withTestRenderer(
+      async (h) => {
+        const shell = createAppShell(h.renderer, {
+          terminal: { columns: 80, rows: 12 },
+          wireKeys: false,
+          run: "idle",
+        })
+        openPalette(shell, { catalog: BIG_CATALOG, title: "commands · /" })
+        await h.renderOnce()
+        for (let i = 0; i < 20; i++) moveOverlaySelection(shell, 1)
+        await h.renderOnce()
+        expect(shell.overlayList?.activeIndex).toBe(20)
+        const offset = shell.overlayList?.offset ?? 0
+        const height = shell.overlayList?.height ?? 0
+        expect(offset).toBeLessThanOrEqual(20)
+        expect(offset + height).toBeGreaterThan(20)
+        const frame = h.captureCharFrame()
+        expect(frame).toContain(`Fake command number 20`)
+      },
+      { width: 80, height: 12 },
+    )
+  })
+})
+