fix(maestro): equalize split widths after spawning a session

splitLeaf always halved just the target pane, so repeated splits left
one pane at 50% and the rest squeezed into shrinking fractions instead
of equal columns.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/layout.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it } from "vitest";
 
 import {
   emptyLayout,
+  equalizeRatios,
   leaves,
   locateSession,
   openSessions,
@@ -44,6 +45,23 @@ describe("splitLeaf", () => {
   });
 });
 
+describe("equalizeRatios", () => {
+  it("resets nested splits to equal width, not just the newest one", () => {
+    const tab = tabWithOneLeaf("s1");
+    const first = leaves(tab.root)[0].id;
+    splitLeaf(tab, first, "s2", "row", true); // A|B, 0.5/0.5
+    const second = leaves(tab.root)[1].id;
+    splitLeaf(tab, second, "s3", "row", true); // A(0.5) | (B|C, 0.5/0.5) -> B,C at 0.25 each
+
+    equalizeRatios(tab.root);
+
+    const root = tab.root as Extract<typeof tab.root, { kind: "split" }>;
+    expect(root.ratio).toBeCloseTo(1 / 3);
+    const inner = root.b as Extract<typeof root.b, { kind: "split" }>;
+    expect(inner.ratio).toBeCloseTo(1 / 2);
+  });
+});
+
 describe("removeLeaf", () => {
   it("collapses the parent split into the surviving sibling", () => {
     const tab = tabWithOneLeaf("s1");

web/src/layout.ts

@@ -118,6 +118,17 @@ export function removeLeaf(tab: Tab, leafId: string): void {
   tab.root = tab.root ? rec(tab.root) : null;
 }
 
+/** Reset every split ratio under `node` so all leaves get equal width,
+ *  regardless of split direction or nesting order. Returns the leaf count. */
+export function equalizeRatios(node: Node | null): number {
+  if (!node) return 0;
+  if (node.kind === "leaf") return 1;
+  const na = equalizeRatios(node.a);
+  const nb = equalizeRatios(node.b);
+  node.ratio = na / (na + nb);
+  return na + nb;
+}
+
 export function addTab(layout: Layout): Tab {
   const tab: Tab = {
     id: freshId("t"),

web/src/multiview.ts

@@ -10,6 +10,7 @@ import {
   activeTab,
   addTab,
   closeTab,
+  equalizeRatios,
   findLeaf,
   freshId,
   type Dir,
@@ -457,6 +458,7 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     dropPreview();
     const tab = activeTab(layout);
     tab.focusedLeafId = splitLeaf(tab, leafId, null, dir, true);
+    equalizeRatios(tab.root);
     render();
   }
 
@@ -542,6 +544,7 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
       target.sessionId = sessionId;
     } else {
       tab.focusedLeafId = splitLeaf(tab, target.id, sessionId, "row", true);
+      equalizeRatios(tab.root);
     }
     render();
     opts.onOpenSessionsChange?.();
@@ -815,6 +818,7 @@ export function createMultiView(opts: MultiViewOptions): MultiView {
     if (source.kind === "pane" && self) removeLeaf(tab, self);
     else if (loc) removeLeaf(loc.tab, loc.leaf.id);
     tab.focusedLeafId = splitLeaf(tab, targetLeafId, sessionId, dir, after);
+    equalizeRatios(tab.root);
     render();
   }