[Xrystal]

And after some more tests with the different action bar change requirements and found this allowed things to work in all scenarios so far found:

nUI_ActionBar:SetAttribute( 
            "_onstate-page", 
            [[
            
                if not newstate then return end
                newstate = tonumber(newstate)
            
                if HasVehicleActionBar() then
                    newPage = GetVehicleBarIndex() or 0
                elseif HasOverrideActionBar() then
                    newPage = GetOverrideBarIndex() or 0
                elseif HasTempShapeshiftActionBar() then
                    newPage = GetTempShapeshiftBarIndex() or 0
                elseif HasBonusActionBar() and GetActionBarPage() == 1 then
                    newPage = GetBonusBarIndex() or 0        
                else
                    newPage = 0
                end
            
                if ( newPage == 0 ) then
                    newstate = GetActionBarPage()
                end
            
        self:SetAttribute( "actionpage", tonumber( newstate ) );
                
            
                -- Update the buttons
                for id = 1, #ActionButtons do
                    local button = ActionButtons[id]
            if not button:GetAttribute( "nUI_ActionButtonOverlay" ) then
            button:SetAttribute( "touch", nil )
                        button:SetAttribute('actionpage', newstate)
                    end
                end
 
        ]]
    );

In addition to these event watching additions

elseif event == "UPDATE_VEHICLE_ACTIONBAR" then
        nUI_ActionBar:Execute(([[   self:SetAttribute('state-page',GetVehicleBarIndex())  ]]))
    elseif event == "UPDATE_OVERRIDE_ACTIONBAR" then
        nUI_ActionBar:Execute(([[   self:SetAttribute('state-page',GetOverrideBarIndex())  ]]))

And for a non nUI setup for those wanting to see the basics in full

local addon,data = ...
data = data or {}
data.BlizzUI = data.BlizzUI or CreateFrame("Frame")
--MainMenuBarArtFrame:SetParent(data.BlizzUI)
data.BlizzUI:Hide()
 
 
local BUTTON_SIZE, BUTTON_SPACING = 50, 1
 
------------------------------------------------------
--[[              Bar Setup                       ]]--
------------------------------------------------------
local barFrame = CreateFrame("Frame", "XBar_Main", UIParent, "SecureHandlerStateTemplate")
barFrame:SetSize((BUTTON_SIZE + BUTTON_SPACING) * NUM_ACTIONBAR_BUTTONS, BUTTON_SIZE)
barFrame:SetPoint("CENTER")
 
local background = barFrame:CreateTexture(nil, "BACKGROUND")
background:SetColorTexture(0, 0, 0, 0.5)
background:SetAllPoints()
 
barFrame:Execute([[
    ActionButtons = newtable()
]])
 
------------------------------------------------------
--[[                 Button Setup                 ]]--
------------------------------------------------------
local buttons = { }
for id = 1, NUM_ACTIONBAR_BUTTONS do
    local button = CreateFrame("CheckButton", "$parent_Button" .. id, barFrame, "ActionBarButtonTemplate")
    button:SetID(id)
    button:SetSize(BUTTON_SIZE, BUTTON_SIZE)
    
    if id ~= 1 then
        button:SetPoint("LEFT", buttons[id - 1], "RIGHT", BUTTON_SPACING, 0)
    else
        button:SetPoint("LEFT", 5, 0)
    end
 
    local background = button:CreateTexture(nil, "BACKGROUND")
    background:SetColorTexture(1, 1, 1, 0.5)
    background:SetAllPoints()
 
    button.NormalTexture:Hide()
 
    barFrame:SetFrameRef("button", button)
    barFrame:Execute(([[
        ActionButtons[%s] = self:GetFrameRef("button")
    ]]):format(id))
 
    buttons[id] = button
end
barFrame.Buttons = buttons
 
------------------------------------------------------
--[[                 Secure Stuff                 ]]--
------------------------------------------------------
barFrame:Execute([[
    self:SetAttribute("frameref-button", nil)
]])
 
barFrame:SetAttribute('_onstate-page', ([[
    if not newstate then return end
    newstate = tonumber(newstate)
        
    if HasVehicleActionBar() then
        newPage = GetVehicleBarIndex() or 0
    elseif HasOverrideActionBar() then
        newPage = GetOverrideBarIndex() or 0
    elseif HasTempShapeshiftActionBar() then
        newPage = GetTempShapeshiftBarIndex() or 0
    elseif HasBonusActionBar() and GetActionBarPage() == 1 then
        newPage = GetBonusBarIndex() or 0    
    else
        newPage = 0
    end
            
    if ( newPage == 0 ) then
        newstate = GetActionBarPage()
    end
        
    self:SetAttribute("actionpage", newstate)
 
    for id = 1, #ActionButtons do
        ActionButtons[id]:SetAttribute('actionpage', newstate)
    end
 
]]):gsub('NUM_ACTIONBAR_BUTTONS', NUM_ACTIONBAR_BUTTONS))
 
 
 
------------------------------------------------------
--[[           Registration Stuff                 ]]--
------------------------------------------------------
RegisterStateDriver(
    barFrame, "page", string.format(
        "[vehicleui][possessbar] %d; [vehicleui] %d; [overridebar] %d; [shapeshift] %d; " ..
        "[bar:2] 2; [bar:3] 3; [bar:4] 4; [bar:5] 5; [bar:6] 6; " ..
        "[bonusbar:1] 7; [bonusbar:2] 8; [bonusbar:3] 9; [bonusbar:4] 10; [bonusbar:5] 11; 1",
        GetVehicleBarIndex(), GetVehicleBarIndex(), GetOverrideBarIndex(), GetTempShapeshiftBarIndex())
);
 
OverrideActionBarLeaveFrameLeaveButton:SetParent(barFrame)
OverrideActionBarLeaveFrameLeaveButton:SetAllPoints(buttons[12])
RegisterStateDriver(OverrideActionBarLeaveFrameLeaveButton, "visibility", "[canexitvehicle] show; hide")
 
local function OnEvent(self,event,...)
    if event == "UPDATE_VEHICLE_ACTIONBAR" then
        barFrame:Execute(([[   self:SetAttribute('state-page',GetVehicleBarIndex())  ]]))
    elseif event == "UPDATE_OVERRIDE_ACTIONBAR" then
        barFrame:Execute(([[   self:SetAttribute('state-page',GetOverrideBarIndex())  ]]))        
    end
end
 
barFrame:RegisterEvent("UPDATE_VEHICLE_ACTIONBAR")
barFrame:RegisterEvent("UPDATE_OVERRIDE_ACTIONBAR")
barFrame:SetScript("OnEvent",OnEvent)