fix(tui): keep streaming progress stable during interaction

ui-tui/src/app/scroll.ts

@@ -0,0 +1,58 @@
+import type { ScrollBoxHandle } from '@hermes/ink'
+
+import type { SelectionApi } from './interfaces.js'
+import { markScrolling } from './interactionMode.js'
+
+export interface SelectionSnap {
+  anchor?: { row: number } | null
+  focus?: { row: number } | null
+  isDragging?: boolean
+}
+
+export interface ScrollWithSelectionOptions {
+  readonly scrollRef: { readonly current: ScrollBoxHandle | null }
+  readonly selection: SelectionApi
+}
+
+export function scrollWithSelectionBy(delta: number, { scrollRef, selection }: ScrollWithSelectionOptions): void {
+  const s = scrollRef.current
+
+  if (!s) {
+    return
+  }
+
+  const cur = s.getScrollTop() + s.getPendingDelta()
+  const viewport = Math.max(0, s.getViewportHeight())
+  const max = Math.max(0, s.getScrollHeight() - viewport)
+  const actual = Math.max(0, Math.min(max, cur + delta)) - cur
+
+  if (actual === 0) {
+    return
+  }
+
+  markScrolling()
+
+  const sel = selection.getState() as null | SelectionSnap
+  const top = s.getViewportTop()
+  const bottom = top + viewport - 1
+
+  if (
+    sel?.anchor &&
+    sel.focus &&
+    sel.anchor.row >= top &&
+    sel.anchor.row <= bottom &&
+    (sel.isDragging || (sel.focus.row >= top && sel.focus.row <= bottom))
+  ) {
+    const shift = sel.isDragging ? selection.shiftAnchor : selection.shiftSelection
+
+    if (actual > 0) {
+      selection.captureScrolledRows(top, top + actual - 1, 'above')
+    } else {
+      selection.captureScrolledRows(bottom + actual + 1, bottom, 'below')
+    }
+
+    shift(-actual, top, bottom)
+  }
+
+  s.scrollBy(actual)
+}