Narrow the side gutter shared by every shell surface

The gutter held at two columns per side from 60 columns wide all the way
up, never scaling down or up with the terminal, which read as excess air
on a wide pane. Drop it to one column, still enough to keep content off
the frame edge at every width the gutter is affordable.

Author: TheGreatAxios

docs/TUI.md

@@ -36,6 +36,15 @@ goal/task/agents strips, then progress, then the prompt itself shrinks one
 row at a time down to its 3-row base — never the transcript
 (`COLLAPSE_ORDER` in `zones.ts`).
 
+Horizontally, every surface sits inside one shared gutter
+(`resolveSideMargin`, `src/tui-opentui/geometry/margins.ts`) so the shell reads
+as a single column of content rather than stacked panes. The gutter is one
+column per side at every width that can afford it, and zero below
+`MARGIN_MIN_COLUMNS` (40), where every column belongs to content. There is no
+middle tier: one column is already enough to keep content off the frame edge,
+which is the gutter's entire job, and anything wider only read as excess air on
+a wide pane. The gutter costs no rows.
+
 The prompt box's border carries the metadata that would otherwise cost a
 titlebar row: the model label sits right-aligned in the top rule; the brand
 lockup sits at the left of the bottom rule with the working directory and git

src/tui-opentui/geometry/index.ts

@@ -21,9 +21,7 @@ export {
 export {
   BOTTOM_MARGIN_MIN_ROWS,
   BOTTOM_MARGIN_ROWS,
-  MARGIN_FULL_MIN_COLUMNS,
   MARGIN_MIN_COLUMNS,
-  NARROW_SIDE_MARGIN,
   SIDE_MARGIN,
   TOP_PAD_MIN_TRANSCRIPT_ROWS,
   TOP_PAD_ROWS,

src/tui-opentui/geometry/margins.ts

@@ -9,24 +9,24 @@
  * here can take a row away from it.
  */
 
-/** Gutter columns on each side once the terminal can afford them. */
-export const SIDE_MARGIN = 2
-
-/** Half gutter for terminals too narrow to spend four columns on air. */
-export const NARROW_SIDE_MARGIN = 1
-
-/** At or above this width the full gutter is affordable. */
-export const MARGIN_FULL_MIN_COLUMNS = 60
+/**
+ * Gutter columns on each side once the terminal can afford them.
+ *
+ * One column at every width the gutter exists at all. A single column is
+ * already enough to keep content off the frame edge, which is the whole job,
+ * and a wider gutter only read as excess air on a wide pane. There is no
+ * middle tier: a width that can spare a column gets one, and a width that
+ * cannot gets none.
+ */
+export const SIDE_MARGIN = 1
 
 /** Below this width every column belongs to content: the gutter goes to zero. */
 export const MARGIN_MIN_COLUMNS = 40
 
 /** Gutter width for a terminal of `columns` columns. */
 export function resolveSideMargin(columns: number): number {
   const cols = Math.max(0, Math.floor(columns))
-  if (cols >= MARGIN_FULL_MIN_COLUMNS) return SIDE_MARGIN
-  if (cols >= MARGIN_MIN_COLUMNS) return NARROW_SIDE_MARGIN
-  return 0
+  return cols >= MARGIN_MIN_COLUMNS ? SIDE_MARGIN : 0
 }
 
 /** Columns left for content after both gutters. */

src/tui-opentui/landing.test.ts

@@ -305,7 +305,7 @@ describe("landing screen", () => {
         expect(ruleRow).toBe(SIZE.height - 1)
         const row = painted[ruleRow]!
         // Left end of the rule, inside the shell gutter, costing no row.
-        expect(row.startsWith("  ╰─ ")).toBe(true)
+        expect(row.startsWith(" ╰─ ")).toBe(true)
         expect(row.trimEnd().endsWith("╯")).toBe(true)
       } finally {
         shell.dispose()

src/tui-opentui/margins.test.ts

@@ -6,9 +6,7 @@
 import { describe, expect, test } from "bun:test"
 import {
   BOTTOM_MARGIN_MIN_ROWS,
-  MARGIN_FULL_MIN_COLUMNS,
   MARGIN_MIN_COLUMNS,
-  NARROW_SIDE_MARGIN,
   SIDE_MARGIN,
   resolveBottomMarginRows,
   resolveContentWidth,
@@ -29,15 +27,22 @@ function frameRows(h: Harness): readonly string[] {
 }
 
 describe("side margin resolution", () => {
-  test("steps down with width and floors at zero", () => {
-    expect(resolveSideMargin(120)).toBe(SIDE_MARGIN)
-    expect(resolveSideMargin(MARGIN_FULL_MIN_COLUMNS)).toBe(SIDE_MARGIN)
-    expect(resolveSideMargin(MARGIN_FULL_MIN_COLUMNS - 1)).toBe(
-      NARROW_SIDE_MARGIN,
-    )
-    expect(resolveSideMargin(MARGIN_MIN_COLUMNS)).toBe(NARROW_SIDE_MARGIN)
+  test("one column at every affordable width, and zero below the floor", () => {
+    expect(SIDE_MARGIN).toBe(1)
+    for (const columns of [MARGIN_MIN_COLUMNS, 60, 80, 120, 200]) {
+      expect(resolveSideMargin(columns)).toBe(1)
+    }
     expect(resolveSideMargin(MARGIN_MIN_COLUMNS - 1)).toBe(0)
     expect(resolveSideMargin(10)).toBe(0)
+    expect(resolveSideMargin(0)).toBe(0)
+  })
+
+  test("content never touches the first or last column of the frame", () => {
+    // The gutter's whole job. A one-column gutter is the narrowest thing that
+    // can do it, so this is what would break if it were ever spent.
+    for (const columns of [80, 120, 200]) {
+      expect(resolveContentWidth(columns)).toBe(columns - 2)
+    }
   })
 
   test("content width never collapses below one column", () => {