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

Investigation 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 without a second windowing path.

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 },
+    )
+  })
+})
+