feat(split-pane): add pure divider-clamp and picker-list helpers

src/web/public/constants.js

@@ -1516,8 +1516,33 @@ function terminalLogicalLine(buffer, row, cols, maxRows) {
   return { startRow, endRow, text, offsetToCell, cellToOffset };
 }
 
+// ═══════════════════════════════════════════════════════════════
+// Split-Pane Sessions — pure helpers (divider math, picker list)
+// ═══════════════════════════════════════════════════════════════
+
+function clampDividerPercent(rawPercent, min = 20, max = 80) {
+  if (rawPercent < min) return min;
+  if (rawPercent > max) return max;
+  return rawPercent;
+}
+
+function buildSplitPickerSessions(sessions, sessionOrder, excludeId) {
+  const result = [];
+  for (const id of sessionOrder) {
+    if (id === excludeId) continue;
+    const session = sessions.get(id);
+    if (!session) continue;
+    result.push({ id, label: session.name || 'Session' });
+  }
+  return result;
+}
+
 if (typeof window !== 'undefined') {
   window.CodemanHistoryFormat = { formatHistoryBytes, computeHistoryTruncationNotice, computeRewriteScrollLine };
   window.CodemanFilePaths = { absoluteFilePathPattern, previewsInFileViewer, FILE_PREVIEW_EXTENSIONS };
   window.CodemanTerminalLines = { terminalLogicalLine };
+  window.CodemanSplitPane = {
+    clampDividerPercent,
+    buildSplitPickerSessions,
+  };
 }

test/split-pane-helpers.test.ts

@@ -0,0 +1,67 @@
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import vm from 'node:vm';
+import { describe, expect, it } from 'vitest';
+
+function loadSplitPaneHelper() {
+  const context = vm.createContext({ window: {}, globalThis: {} });
+  const source = readFileSync(resolve(import.meta.dirname, '../src/web/public/constants.js'), 'utf8');
+  vm.runInContext(source, context, { filename: 'constants.js' });
+  return (context.window as { CodemanSplitPane: any }).CodemanSplitPane;
+}
+
+describe('CodemanSplitPane.clampDividerPercent', () => {
+  it('passes through a value inside the clamp range', () => {
+    const { clampDividerPercent } = loadSplitPaneHelper();
+    expect(clampDividerPercent(50)).toBe(50);
+    expect(clampDividerPercent(35.5)).toBe(35.5);
+  });
+
+  it('clamps below the floor to the floor', () => {
+    const { clampDividerPercent } = loadSplitPaneHelper();
+    expect(clampDividerPercent(5)).toBe(20);
+  });
+
+  it('clamps above the ceiling to the ceiling', () => {
+    const { clampDividerPercent } = loadSplitPaneHelper();
+    expect(clampDividerPercent(95)).toBe(80);
+  });
+
+  it('honors custom min/max', () => {
+    const { clampDividerPercent } = loadSplitPaneHelper();
+    expect(clampDividerPercent(10, 15, 85)).toBe(15);
+    expect(clampDividerPercent(90, 15, 85)).toBe(85);
+  });
+});
+
+describe('CodemanSplitPane.buildSplitPickerSessions', () => {
+  it('excludes the active session and preserves tab order', () => {
+    const { buildSplitPickerSessions } = loadSplitPaneHelper();
+    const sessions = new Map([
+      ['a', { name: 'w1-codeman' }],
+      ['b', { name: 'w1-mcp-memory' }],
+      ['c', { name: null }],
+    ]);
+    const sessionOrder = ['a', 'b', 'c'];
+    const result = buildSplitPickerSessions(sessions, sessionOrder, 'a');
+    expect(result).toEqual([
+      { id: 'b', label: 'w1-mcp-memory' },
+      { id: 'c', label: 'Session' },
+    ]);
+  });
+
+  it('drops order entries with no matching session (stale ids)', () => {
+    const { buildSplitPickerSessions } = loadSplitPaneHelper();
+    const sessions = new Map([['a', { name: 'w1-codeman' }]]);
+    const sessionOrder = ['a', 'ghost'];
+    const result = buildSplitPickerSessions(sessions, sessionOrder, null);
+    expect(result).toEqual([{ id: 'a', label: 'w1-codeman' }]);
+  });
+
+  it('returns an empty list when only the excluded session exists', () => {
+    const { buildSplitPickerSessions } = loadSplitPaneHelper();
+    const sessions = new Map([['a', { name: 'w1-codeman' }]]);
+    const result = buildSplitPickerSessions(sessions, ['a'], 'a');
+    expect(result).toEqual([]);
+  });
+});