[Cr4x]

As Errc said you can watch EVENT_ACTION_LAYERS_POPPED and _PUSHED for just watching if something is opened.

If you need to access further informations for example when the Inventory is opened:

MyAddon = {}
MyAddon.InventoryOpened = false
 
function MyAddon.ZOPlayerInventoryOnShow(self, hidden)
    if ( not hidden ) then
        MyAddon.InventoryOpened = true
    end
end
 
function MyAddon.ZOPlayerInventoryOnHide(self, hidden)
    if ( hidden ) then
        MyAddon.InventoryOpened = false
    end
end
 
function MyAddon.OnAddOnLoaded(eventcode, addOnName)
    if (addOnName ~= "MyAddon") then return end
 
    -- ZO_PlayerInventory < ToplevelControl which is shown when pressing "i"
    ZO_PlayerInventory:SetHandler("OnShow", MyAddon.ZOPlayerInventoryOnShow)
    ZO_PlayerInventory:SetHandler("OnShow", MyAddon.ZOPlayerInventoryOnHide)
end
 
EVENT_MANAGER:RegisterForEvent("MyAddon_EVENT_ADD_ON_LOADED", EVENT_ADD_ON_LOADED, MyAddon.OnAddOnLoaded)