https://forum.cockos.com/showthread.php?p=2179860

cfillion_Save and compare opened project tabs.lua

-- @version 1.0beta
-- @author cfillion

local EXT_SECTION = 'cfillion_project_list'

local function makeKey(i)
  return string.format('project%d', i)
end

local function enumOpenedProjects()
  local i = 0
  
  return function()
    local retval, projfn = reaper.EnumProjects(i)
    i = i + 1

    if retval then
      return i, projfn
    end
  end
end

local function getOpenedProjects()
  local list = {}
  
  for i, projfn in enumOpenedProjects() do
    table.insert(list, projfn)
  end
  
  return list
end

local function saveOpenedProjects()
  for i, projfn in enumOpenedProjects() do
    reaper.SetExtState(EXT_SECTION, makeKey(i), projfn, true)
  end
end

local function enumSavedProjects()
  local i = 1
  
  return function()
    local key = makeKey(i)
    i = i + 1

    if reaper.HasExtState(EXT_SECTION, key) then
      return key
    end
  end
end

local function getSavedProjects()
  local list = {}

  for key in enumSavedProjects() do
    local projfn = reaper.GetExtState(EXT_SECTION, key)
    table.insert(list, projfn)
  end
  
  return list
end

local function clearSavedProjects()
  for key in enumSavedProjects() do
    reaper.DeleteExtState(EXT_SECTION, key, true)
  end
end

local function isDifferent(tableA, tableB)
  if #tableA ~= #tableB then
    return true
  end
  
  for i, val in ipairs(tableA) do
    if tableB[i] ~= val then
      return true
    end
  end
  
  return false
end

local function compareLists()
  local savedProjects = getSavedProjects()
  local openedProjects = getOpenedProjects()
  
  if isDifferent(savedProjects, openedProjects) then
    reaper.MB("The saved projects are different from the currently opened ones!", "Test", 0)
  end
end

--compareLists()
--clearSavedProjects()
--saveOpenedProjects()

At line 14, is it normal that there's no 2nd argument to the enumproject function?
Shouldn't it be:
reaper.EnumProjects(i,'')

It is no longer necessary since v5.982 (though the change was not mentioned in the changelog?). As seen in the documentation:

v5.981

Lua: ReaProject retval, string projfn = reaper.EnumProjects(integer idx, string projfn)

v5.982

Lua: ReaProject retval, optional string projfn = reaper.EnumProjects(integer idx)

Thanks for this. I didn't know.
That explains why it was working for me and not for others.