diff --git a/packages/app/e2e/browser/split-pane-resize.spec.ts b/packages/app/e2e/browser/split-pane-resize.spec.ts
new file mode 100644
index 0000000000..75414ee321
--- /dev/null
+++ b/packages/app/e2e/browser/split-pane-resize.spec.ts
@@ -0,0 +1,88 @@
+import { expect, type Locator, type Page } from "@playwright/test";
+import { test } from "../support/fixtures";
+import { runWorkspaceActionFromCommandCenter } from "../support/helpers/command-center-workspace-actions";
+import { clickNewChat, gotoWorkspace } from "../support/helpers/launcher";
+import { seedWorkspace } from "../support/helpers/seed-client";
+import { waitForWorkspaceTabsVisible } from "../support/helpers/workspace-tabs";
+
+const DRAG_DISTANCE = -40;
+
+function splitDividers(page: Page): Locator {
+  return page.getByTestId("workspace-split-resize-handle").filter({ visible: true });
+}
+
+async function dividerX(divider: Locator): Promise<number> {
+  const box = await divider.boundingBox();
+  if (!box) throw new Error("Divider has no bounding box");
+  return box.x;
+}
+
+async function dragDivider(page: Page, divider: Locator, deltaX: number): Promise<void> {
+  const box = await divider.boundingBox();
+  if (!box) throw new Error("Divider has no bounding box");
+  const x = box.x + box.width / 2;
+  const y = box.y + box.height / 2;
+  await page.mouse.move(x, y);
+  await page.mouse.down();
+  await page.mouse.move(x + deltaX / 2, y, { steps: 4 });
+  await page.mouse.move(x + deltaX, y, { steps: 4 });
+  await page.mouse.up();
+}
+
+async function expectDividerFollowsDrag(page: Page, index: number): Promise<void> {
+  const divider = splitDividers(page).nth(index);
+  const before = await dividerX(divider);
+  await dragDivider(page, divider, DRAG_DISTANCE);
+  await expect
+    .poll(async () => (await dividerX(divider)) - before, {
+      message: `divider ${index + 1} should follow the drag`,
+    })
+    .toBeLessThan(DRAG_DISTANCE * 0.75);
+}
+
+async function splitRight(page: Page, expectedDividers: number): Promise<void> {
+  await runWorkspaceActionFromCommandCenter(page, "Split pane right");
+  await expect(splitDividers(page)).toHaveCount(expectedDividers);
+}
+
+test.describe("Split pane resize", () => {
+  test("every divider drags in a four-pane row", async ({ page }) => {
+    await page.setViewportSize({ width: 1600, height: 900 });
+    const workspace = await seedWorkspace({ repoPrefix: "split-resize-fresh-" });
+    try {
+      await gotoWorkspace(page, workspace.workspaceId);
+      await waitForWorkspaceTabsVisible(page);
+      await clickNewChat(page);
+      await splitRight(page, 1);
+      await splitRight(page, 2);
+      await splitRight(page, 3);
+
+      for (const index of [0, 1, 2]) {
+        await expectDividerFollowsDrag(page, index);
+      }
+    } finally {
+      await workspace.cleanup();
+    }
+  });
+
+  test("every divider drags after panes are added to a resized row", async ({ page }) => {
+    await page.setViewportSize({ width: 1600, height: 900 });
+    const workspace = await seedWorkspace({ repoPrefix: "split-resize-grown-" });
+    try {
+      await gotoWorkspace(page, workspace.workspaceId);
+      await waitForWorkspaceTabsVisible(page);
+      await clickNewChat(page);
+      await splitRight(page, 1);
+      await expectDividerFollowsDrag(page, 0);
+
+      await splitRight(page, 2);
+      await splitRight(page, 3);
+
+      for (const index of [0, 1, 2]) {
+        await expectDividerFollowsDrag(page, index);
+      }
+    } finally {
+      await workspace.cleanup();
+    }
+  });
+});
diff --git a/packages/app/src/components/split-container.tsx b/packages/app/src/components/split-container.tsx
index 9705f3be15..7f627a8c72 100644
--- a/packages/app/src/components/split-container.tsx
+++ b/packages/app/src/components/split-container.tsx
@@ -872,6 +872,19 @@ function SplitGroupChild({
   );
 }
 
+/**
+ * Sizes from a divider drag are kept per group id, apart from the layout tree. A group keeps its
+ * id when panes are added or removed, so the dragged sizes only apply while they still have one
+ * entry per child; otherwise the dividers past the end of the list could not move.
+ */
+function resolveGroupSizes(
+  layoutSizes: number[],
+  draggedSizes: number[] | undefined,
+  childCount: number,
+): number[] {
+  return draggedSizes?.length === childCount ? draggedSizes : layoutSizes;
+}
+
 /**
  * Flex grow per child, renormalized so the visible ones always sum to 1.
  *
@@ -968,8 +981,11 @@ function SplitNodeView({
     groupId ? state.splitSizesByWorkspace[workspaceKey]?.[groupId] : undefined,
   );
   const groupChildren = node.kind === "group" ? node.group.children : EMPTY_SPLIT_NODES;
-  const groupSizes =
-    storedGroupSizes ?? (node.kind === "group" ? node.group.sizes : EMPTY_SPLIT_SIZES);
+  const groupSizes = resolveGroupSizes(
+    node.kind === "group" ? node.group.sizes : EMPTY_SPLIT_SIZES,
+    storedGroupSizes,
+    groupChildren.length,
+  );
   const visibleFlex = useMemo(
     () => resolveVisibleGroupFlex(groupChildren, groupSizes, maximizedPaneId),
     [groupChildren, groupSizes, maximizedPaneId],