perf(tui): stabilize long-session scrolling

ui-tui/src/lib/inputMetrics.ts

@@ -0,0 +1,62 @@
+import { stringWidth } from '@hermes/ink'
+
+let _seg: Intl.Segmenter | null = null
+const seg = () => (_seg ??= new Intl.Segmenter(undefined, { granularity: 'grapheme' }))
+
+/**
+ * Mirrors the char-wrap behavior used by the composer TextInput.
+ * Returns the zero-based visual line and column of the cursor cell.
+ */
+export function cursorLayout(value: string, cursor: number, cols: number) {
+  const pos = Math.max(0, Math.min(cursor, value.length))
+  const w = Math.max(1, cols)
+
+  let col = 0,
+    line = 0
+
+  for (const { segment, index } of seg().segment(value)) {
+    if (index >= pos) {
+      break
+    }
+
+    if (segment === '\n') {
+      line++
+      col = 0
+
+      continue
+    }
+
+    const sw = stringWidth(segment)
+
+    if (!sw) {
+      continue
+    }
+
+    if (col + sw > w) {
+      line++
+      col = 0
+    }
+
+    col += sw
+  }
+
+  // trailing cursor-cell overflows to the next row at the wrap column
+  if (col >= w) {
+    line++
+    col = 0
+  }
+
+  return { column: col, line }
+}
+
+export function inputVisualHeight(value: string, columns: number) {
+  return cursorLayout(value, value.length, columns).line + 1
+}
+
+export function stableComposerColumns(totalCols: number, promptWidth: number) {
+  // totalCols is the terminal width. Reserve:
+  // - outer composer paddingX={1}: 2 columns
+  // - transcript scrollbar gutter + marginLeft: 2 columns
+  // - prompt prefix width
+  return Math.max(20, totalCols - promptWidth - 4)
+}