Function dockTabToEdge

src/browser/utils/rightSidebarLayout.ts:688–800

(
  state: RightSidebarLayoutState,
  tab: TabType,
  sourceTabsetId: string,
  targetTabsetId: string,
  edge: TabDockEdge
)

 * - avoiding empty tabsets when a user drags out the last remaining tab
 */
export function dockTabToEdge(
  state: RightSidebarLayoutState,
  tab: TabType,
  sourceTabsetId: string,
  targetTabsetId: string,
  edge: TabDockEdge
): RightSidebarLayoutState {
  const source = findTabset(state.root, sourceTabsetId);
  const target = findTabset(state.root, targetTabsetId);

  if (source?.type !== "tabset" || target?.type !== "tabset") {
    return state;
  }

  if (!source.tabs.includes(tab)) {
    return state;
  }

  const splitDirection: "horizontal" | "vertical" =
    edge === "top" || edge === "bottom" ? "horizontal" : "vertical";
  const insertBefore = edge === "top" || edge === "left";

  const splitAlloc = allocId(state, "split");
  const tabsetAlloc = allocId({ ...state, nextId: splitAlloc.nextId }, "tabset");

  const newTabset: Extract<RightSidebarLayoutNode, { type: "tabset" }> = {
    type: "tabset",
    id: tabsetAlloc.id,
    tabs: [tab],
    activeTab: tab,
  };

  const updateNode = (node: RightSidebarLayoutNode): RightSidebarLayoutNode | null => {
    if (node.type === "tabset") {
      if (node.id === targetTabsetId) {
        let updatedTarget = node;

        // When dragging out of this tabset, remove the tab before splitting.
        if (sourceTabsetId === targetTabsetId) {
          const remaining = node.tabs.filter((t) => t !== tab);
          const fallbackTab = getFallbackTabForEmptyTabset(tab);
          const nextTabs = remaining.length > 0 ? remaining : [fallbackTab];
          const nextActiveTab =
            node.activeTab === tab || !nextTabs.includes(node.activeTab)
              ? nextTabs[0]
              : node.activeTab;
          updatedTarget = { ...node, tabs: nextTabs, activeTab: nextActiveTab };
        }

        const children: [RightSidebarLayoutNode, RightSidebarLayoutNode] = insertBefore
          ? [newTabset, updatedTarget]
          : [updatedTarget, newTabset];

        return {
          type: "split",
          id: splitAlloc.id,
          direction: splitDirection,
          sizes: [50, 50],