Javascript hack to fix invisible (Visible="false") AjaxControlToolkit TabPanels.

Default.aspx

<asp:TabPanel runat="server" ID="secondTab" HeaderText="2nd" Visible="false">

Hack.js

_selectedPanelsId = {}

var setVisibility = function(panelId) {
  // Header.
  var activeHeader = $('span[tab-panel='+ panelId +']');
  var headers = activeHeader.parent().children()
  headers.removeClass('ajax__tab_active')
  activeHeader.addClass('ajax__tab_active')
  
  // Panels.
  var activePanel = $('#'+ panelId);
  var panels = activePanel.parent().children()
  panels.css('visibility', 'hidden').hide()
  activePanel.css('visibility', 'visible').show()
}

var tabHack = function() {
  setTimeout(function() {
    var containers = $('.ajax__tab_container')

    // Foreach tab header...
    containers.each(function() {
      var container = $(this)
      var containerId = container.get(0).id
      var headers = container.find('.ajax__tab_header').first().children()
      var panels = container.find('.ajax__tab_body').first().children()
      var panelIndex = 0;

      // Foreach tabHeader...
      headers.each(function() {
        var header = $(this)
        var enabled = header.children().length > 0;
        
        // Add disabled behavior.
        if (!enabled) {
          header.replaceWith('<span class="disabled"><span class="ajax__tab_outer"><span class="ajax__tab_inner"><span class="ajax__tab_tab">'+ header.text() +'</span></span></span></span>')
        }

        // Fix onclick bug.
        if(enabled && panels[panelIndex++] != null) {
          header.attr('tab-panel', panels[panelIndex++].id)
          header.click(function() {
            var panelId = $(this).attr('tab-panel')
            setVisibility(panelId)
            _selectedPanelsId[containerId] = panelId
          })
        }
      })

      // Fix ajax postback tabindex bug.
      var selectedPanelId = _selectedPanelsId[containerId]
      if(selectedPanelId != null) {
        setVisibility(selectedPanelId)
      }

    })
  }, 1)
}

Sys.Application.add_init(tabHack)
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(tabHack)

Style.css

.CWITabContainer .disabled .ajax__tab_tab {
	background: url(../Images/CWITabContainer/tab-disabled.gif) repeat-x;
	cursor: default;
}

.CWITabContainer .disabled .ajax__tab_outer {
	background: url(../Images/CWITabContainer/tab-disabled-right.gif) no-repeat right;
}

.CWITabContainer .disabled .ajax__tab_inner {
	background: url(../Images/CWITabContainer/tab-disabled-left.gif) no-repeat;
}