Function closeSplit

src/browser/utils/rightSidebarLayout.ts:808–842

(
  state: RightSidebarLayoutState,
  splitId: string,
  keepChildIndex: 0 | 1
)

 * @param keepChildIndex Which child to keep (0 = first/left/top, 1 = second/right/bottom)
 */
export function closeSplit(
  state: RightSidebarLayoutState,
  splitId: string,
  keepChildIndex: 0 | 1
): RightSidebarLayoutState {
  const replaceNode = (node: RightSidebarLayoutNode): RightSidebarLayoutNode => {
    if (node.type === "tabset") {
      return node;
    }

    if (node.id === splitId) {
      // Replace this split with the kept child
      return node.children[keepChildIndex];
    }

    return {
      ...node,
      children: [replaceNode(node.children[0]), replaceNode(node.children[1])],
    };
  };

  const newRoot = replaceNode(state.root);

  // Ensure focusedTabsetId is still valid
  let newFocusedId: string = state.focusedTabsetId;
  if (findTabset(newRoot, newFocusedId) === null) {
    newFocusedId = findFirstTabsetId(newRoot) ?? state.focusedTabsetId;
  }

  return {
    ...state,
    focusedTabsetId: newFocusedId,
    root: newRoot,
  };
}