fix(tui): preserve composer width on narrow panes

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

@@ -42,9 +42,9 @@ describe('input metrics helpers', () => {
     expect(inputVisualHeight('one\ntwo', 40)).toBe(2)
   })
 
-  it('reserves gutters without exceeding the physical terminal width', () => {
+  it('reserves gutters on wide panes without starving narrow composer width', () => {
     expect(stableComposerColumns(100, 3)).toBe(93)
-    expect(stableComposerColumns(10, 3)).toBe(3)
+    expect(stableComposerColumns(10, 3)).toBe(5)
     expect(stableComposerColumns(6, 3)).toBe(1)
   })
 })

ui-tui/src/lib/inputMetrics.ts

@@ -54,9 +54,9 @@ export function inputVisualHeight(value: string, columns: number) {
 }
 
 export function stableComposerColumns(totalCols: number, promptWidth: number) {
-  // Physical render/wrap width. Reserve:
-  // - outer composer paddingX={1}: 2 columns
-  // - transcript scrollbar gutter + marginLeft: 2 columns
-  // - prompt prefix width
-  return Math.max(1, totalCols - promptWidth - 4)
+  // Physical render/wrap width. Always reserve outer composer padding and
+  // prompt prefix. Only reserve the transcript scrollbar gutter when the
+  // terminal is wide enough; on narrow panes, preserving input columns beats
+  // keeping gutters visually aligned.
+  return Math.max(1, totalCols - promptWidth - 2 - (totalCols - promptWidth >= 24 ? 2 : 0))
 }