Sliding Example

-- Simple variables
local screenX, screenY = term.getSize()
local status_user = "Idle"
local stutus_connection = "Not Connected"
local last_clicked

-- Timers
local updateTime = os.startTimer( 1 )

-- Load the background image
-- Sidebar settings
local sideBarOut = false
local sidebar_text = {
    "Message",
    "-------",
    "New",
    "Archive",
    "Delete", "",
    "Folder",
    "-------",
    "Inbox",
    "Sent",
    "Archive", "",
    "Options",
    "-------",
    "UserID",
    "Logout"
}

-- Functions
-- Functions from my ComputerCraft YouTube Program
local function centerWrite( text, yPos, txtCol, bgCol )
    term.setCursorPos( ( screenX - #text ) / 2, yPos )
    term.setTextColour( txtCol )
    term.setBackgroundColour( bgCol )
    write( text )
end

local function leftWrite( text, yPos, txtCol, bgCol )
    term.setCursorPos( 1, yPos )
    term.setTextColour( txtCol )
    term.setBackgroundColour( bgCol )
    write( text )
end

local function rightWrite( text, yPos, txtCol, bgCol )
    term.setCursorPos( screenX - #text + 1, yPos )
    term.setTextColour( txtCol )
    term.setBackgroundColour( bgCol )
    write( text )
end

local function sWrite( text, xPos, yPos, txtCol, bgCol )
    term.setCursorPos( xPos, yPos )
    term.setTextColour( txtCol )
    term.setBackgroundColour( bgCol )
    write( text )
end

local function transEffect( s )
    if s == "out" then
        term.setBackgroundColour( colours.cyan )
        for x = 1, 9 do
            term.setBackgroundColour( colours.cyan )
            for y = 3, screenY - 1 do
                term.setCursorPos( x, y )
                write( ' ' )
            end
            term.setTextColour( colours.black )
            for i = 1, #sidebar_text do
                term.setCursorPos( 2, i + 2 )
                write( sidebar_text[i]:sub( math.abs( x - 10 ) ) )
            end
            sleep( 0.05 )
        end
    elseif s == "in" then
        term.setTextColour( colours.white )
        for x = 10, 2, -1 do
            for y = 3, screenY - 1 do
                term.setBackgroundColour( y == 3 and colours.blue or colours.black )
                term.setCursorPos( x, y )
                write( ( x == 10 and y == 3 ) and 'j' or ( x == 9 and y == 3 ) and 'b' or ( x == 8 and y == 3 ) and 'u' or ( x == 7 and y == 3 ) and 'S' or ' ' )
            end
            sleep( 0.05 )
        end
    end
    term.setBackgroundColour( colours.red )
    term.setTextColour( colours.white )
    for y = 3, screenY - 1 do
        term.setCursorPos( s == "out" and 10 or 1, y )
        write( y == math.floor( ( 3 + ( screenY - 1 ) ) / 2 ) and (s == "out" and '<' or '>') or ' ' )
    end
end

-- Other functions
local function drawBG( t )
    term.clear()
    paintutils.drawImage( t, 1, 1 )

    -- Header
    centerWrite( "SmartMail Client", 1, colours.red, colours.orange )
    leftWrite( "Computer ID: " .. os.getComputerID(), 2, colours.red, colours.orange )
    rightWrite( "User ID: " .. os.getComputerID(), 2, colours.red, colours.orange )     -- Not sure what the difference between user ID and computer ID as they are the same in your screenshot :P

    -- Sidebar '>'
    if not sideBarOut then sWrite( ">", 1, math.floor( ( 3 + ( screenY - 1 ) ) / 2 ), colours.white, colours.red ) end

    -- Email table section
    sWrite( "     Subject       |    From     |    Received    ", 2, 3, colours.white, colours.blue )

    -- Footer
    leftWrite( "Status: " .. status_user, screenY, colours.white, colours.blue )
    centerWrite( "Status: " .. stutus_connection, screenY, colours.white, colours.blue )
    rightWrite( textutils.formatTime( os.time(), true ), screenY, colours.white, colours.blue )
end

local function checkClick( tab, cx, cy, sx, ex, sy )
    for i, v in pairs( tab ) do
        if cx >= sx and cx <= ex and cy == sy + i - 1 then
            return true, v
        end
    end
    return false
end

-- Main
drawBG( bg )

parallel.waitForAny(
    function()
        while true do
            sWrite( "     Sidebar: " .. ( sideBarOut and 'out' or 'in ' ), 15, 5, colours.white, colours.black )
            sWrite( "Last clicked: " .. ( last_clicked or 'none' ) .. '    ', 15, 6, colours.white, colours.black ) -- The extra spaces to cut out text from previous writes
            local ev = { os.pullEvent() }
            if ev[1] == "mouse_click" and ev[2] == 1 then
                if ev[3] == 1 and not sideBarOut then
                    transEffect( 'out' )
                    sideBarOut = not sideBarOut
                elseif ev[3] == 10 and sideBarOut then
                    transEffect( 'in' )
                    sideBarOut = not sideBarOut
                elseif sideBarOut then
                    local bValid, clicked = checkClick( sidebar_text, ev[3], ev[4], 1, 10, 3 )
                    if bValid and clicked ~= "" and clicked ~= "-------" then last_clicked = clicked end
                    -- Code would be here to check what was clicked
                end
            end
        end
    end,
    function()
        while true do
            local ev = { os.pullEvent() }
            if ev[1] == "timer" and ev[2] == updateTime then
                updateTime = os.startTimer( 1 )
                rightWrite( textutils.formatTime( os.time(), true ), screenY, colours.white, colours.blue )
            end
        end
    end
)