[MunkDev]

-- FrameXML\SecureTemplates.lua:351
SECURE_ACTIONS.pet =
    function (self, unit, button)
        local action =
            SecureButton_GetModifiedAttribute(self, "action", button);
        if ( action ) then
            CastPetAction(action, unit);
        end
    end;

Damn, I didn't even know this. They could change it to this...

SECURE_ACTIONS.pet =
    function (self, unit, button)
        local action =
            SecureButton_GetModifiedAttribute(self, "action", button);
        if ( action ) then
            if ( button == "LeftButton" ) then
                CastPetAction(action, unit);
            else
                TogglePetAutocast(action);
            end
        end
    end;

...or give a separate type for it since you might want to use something other than LeftButton as input.

I've also built my own version of LAB because it didn't cover all my needs, but here are my recommendations anyway:

• I'm just using PetActionButtonTemplate for a separate pet bar. I haven't bothered trying to fit a square peg in a round hole.
  
• For posterity I recommend not pre-calculating the actionpage attribute using a state driver, because to me it seems these numbers may change when they make changes to certain specs; Shadowdance used to have its own page, and monks had several pages for stances that no longer exist.
  
  

  local function GetActionpageStateDriver()
      -- Generate a macro condition with generic values to ensure any change pushes an update.
      -- Add any new / extra macro conditions to the list below. (as if there aren't enough already)
      local conditionFormat = '[%s] %d; '
      local count, driver = 0, ''
      local conditions = {
          ----------------------------------
          'vehicleui', 'possessbar', 'overridebar', 'shapeshift',
          'bar:2', 'bar:3', 'bar:4', 'bar:5', 'bar:6',
          'bonusbar:1', 'bonusbar:2', 'bonusbar:3', 'bonusbar:4'
          ----------------------------------
      }
      for i, condition in ipairs(conditions) do
          driver = driver .. format(conditionFormat, condition, i)
      end
      driver = driver .. (#conditions + 1) -- append the list for the default bar (1) when none of the conditions apply.
      -- this ends up looking like this: "[vehicleui] 1; [possessbar] 2; ... [bonusbar:4] 13; 14"
      ----------------------------------
      return driver
  end
   
  -- Register the generic state driver just so it sends updates on page swaps.
  RegisterStateDriver(mainBar, 'page', GetActionpageStateDriver())
   
  -- With this approach, newstate is expected to return an incorrect value,
  -- because I didn't bother finding out which condition corresponds to which page.
  -- Instead I'm using the available API in the environment to calculate the page.
  -- This is pretty much an exact copy of ActionBarController_UpdateAll,
  -- which you can find in FrameXML\ActionBarController.lua:107
  mainBar:SetAttribute('_onstate-page', [[
      if HasVehicleActionBar() then
          newstate = GetVehicleBarIndex()
      elseif HasOverrideActionBar() then
          newstate = GetOverrideBarIndex()
      elseif HasTempShapeshiftActionBar() then
          newstate = GetTempShapeshiftBarIndex()
      elseif GetBonusBarOffset() > 0 then
          newstate = GetBonusBarOffset() + 6
      else
          newstate = GetActionBarPage()
      end
      self:SetAttribute('actionpage', newstate)
      control:ChildUpdate('actionpage', newstate)
  ]])