모델로 보는 탭과 창

    private func insert(_ tab: BrowserTab, after opener: BrowserTab?) {
        tab.window = self
        if let opener, let index = index(of: opener.id) {
            // A tab opened from a pinned tab lands after the pinned group.
            tabs.insert(tab, at: max(index + 1, pinnedCount))
        } else {
            tabs.append(tab)
        }
    }

    // MARK: Pinning and the tab commands

    /// A pinned tab joins the end of the pinned group; an unpinned one goes
    /// to the front of the rest.
    public func setPinned(_ id: TabID, _ pinned: Bool) {
        guard let index = index(of: id), tabs[index].isPinned != pinned else { return }
        let tab = tabs.remove(at: index)
        tab.isPinned = pinned
        tabs.insert(tab, at: pinnedCount)
        sessionChanged()
    }

    public func close(_ id: TabID) {
        guard let index = index(of: id) else { return }
        let tab = tabs[index]
        let snapshot = tab.snapshot()
        if !isPrivate, !tab.isPrivate, snapshot.url != nil {
            closedTabs.append((snapshot, index))
            if closedTabs.count > Self.closedTabLimit { closedTabs.removeFirst() }
        }
        tab.discardPage()
        tab.window = nil
        tabs.remove(at: index)
        if selectedID == id {
            // The tab to the right takes the place, or the last one.
            let next = tabs.indices.contains(index) ? tabs[index] : tabs.last
            selectedID = next?.id
            next?.wake(using: services.engine)
        }
        sessionChanged()
    }

    @discardableResult
    public func reopenClosedTab() -> BrowserTab? {
        guard let (snapshot, index) = closedTabs.popLast() else { return nil }
        // A new tab id: the closed tab is gone, this is its place reopened.
        let tab = BrowserTab(
            identity: snapshot.identity,
            status: PageStatus(url: snapshot.url, title: snapshot.title, favicon: snapshot.favicon),
            restorable: snapshot.state, pinned: snapshot.pinned)
        tab.sharesContextWithPippa = snapshot.sharesContextWithPippa
        tab.window = self
        let place = snapshot.pinned ? min(index, pinnedCount) : min(max(index, pinnedCount), tabs.count)
        tabs.insert(tab, at: place)
        select(tab.id)
        sessionChanged()
        return tab
    }

    /// Chrome's ⌘1…⌘8 pick a tab by position; ⌘9 is always the last one.
    public func select(position: Int) {
        guard !tabs.isEmpty else { return }
        let index = position >= 9 ? tabs.count - 1 : position - 1
        guard tabs.indices.contains(index) else { return }
        select(tabs[index].id)
    }

    /// A tab moves within its own group: pinned tabs among the pinned, the
    /// rest among the rest.
    public func move(_ id: TabID, to destination: Int) {
        guard let index = index(of: id) else { return }
        let tab = tabs.remove(at: index)
        let pinned = tabs.prefix { $0.isPinned }.count
        let place = tab.isPinned ? min(max(destination, 0), pinned) : min(max(destination, pinned), tabs.count)
        tabs.insert(tab, at: place)
        sessionChanged()
    }