fix(tui): keep streaming progress stable during interaction

ui-tui/src/__tests__/scroll.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, it, vi } from 'vitest'
+
+import { scrollWithSelectionBy } from '../app/scroll.js'
+
+function makeScroll(overrides: Partial<Record<string, unknown>> = {}) {
+  return {
+    getPendingDelta: vi.fn(() => 0),
+    getScrollHeight: vi.fn(() => 100),
+    getScrollTop: vi.fn(() => 10),
+    getViewportHeight: vi.fn(() => 20),
+    getViewportTop: vi.fn(() => 0),
+    scrollBy: vi.fn(),
+    ...overrides
+  }
+}
+
+describe('scrollWithSelectionBy', () => {
+  it('clamps to the actual remaining scroll distance before calling scrollBy', () => {
+    const s = makeScroll({
+      getScrollHeight: vi.fn(() => 30),
+      getScrollTop: vi.fn(() => 9),
+      getViewportHeight: vi.fn(() => 20)
+    })
+    const selection = {
+      captureScrolledRows: vi.fn(),
+      getState: vi.fn(() => null),
+      shiftAnchor: vi.fn(),
+      shiftSelection: vi.fn()
+    }
+
+    scrollWithSelectionBy(10, { scrollRef: { current: s as never }, selection })
+
+    expect(s.scrollBy).toHaveBeenCalledWith(1)
+  })
+
+  it('does nothing at the edge instead of queueing dead pending deltas', () => {
+    const s = makeScroll({
+      getScrollHeight: vi.fn(() => 30),
+      getScrollTop: vi.fn(() => 10),
+      getViewportHeight: vi.fn(() => 20)
+    })
+    const selection = {
+      captureScrolledRows: vi.fn(),
+      getState: vi.fn(() => null),
+      shiftAnchor: vi.fn(),
+      shiftSelection: vi.fn()
+    }
+
+    scrollWithSelectionBy(10, { scrollRef: { current: s as never }, selection })
+
+    expect(s.scrollBy).not.toHaveBeenCalled()
+  })
+})