I don't know.Sometimes the Ctrl gets stuck,even though I didn't see any problem in the key.The icons just turn white,and I keep drawing rectangles.I then have to press Ctrl again to stop it.

I wrote a simple Shift-lock function in Lua for you. Please note: it's very basic, so you must turn the lock off when you want to select elements normally.

if StickyLineDraw then
    return
end
StickyLineDraw = true

local a = false
local b = false
local c, d = 0, 0
local e, f = 0, 0

local function g(h, i, j, k, l, m)
    if not j and (i == 225 or i == 229 or h == 16) then
        a = not a
        tpt.log("Sticky line mode: " .. (a and "ON" or "OFF"))
    end
end

local function n(o, p, q)
    if q == 1 and a then
        b = true
        c, d = o, p
        e, f = o, p
        return false
    end
end

local function r(o, p, s, t)
    if b and a then
        e, f = o, p
    end
end

local function u(o, p, q, v)
    if b and a and q == 1 then
        b = false
        local w, x = tpt.brushx, tpt.brushy
        local y = tpt.brushID
        sim.createLine(c, d, o, p, w, x, nil, y)
        return false
    end
end

local function z()
    if a then
        tpt.drawtext(10, 20, "Line mode locked", 255, 255, 0)
    end
    if b and a then
        local w, x = tpt.brushx, tpt.brushy
        tpt.drawline(c, d, e, f, 255, 255, 255, 120)
        for t = -x, x do
            tpt.drawline(c, d + t, e, f + t, 255, 255, 255, 40)
        end
        for s = -w, w do
            tpt.drawline(c + s, d, e + s, f, 255, 255, 255, 40)
        end
        tpt.fillrect(c - w, d - x, w * 2, x * 2, 0, 255, 0, 100)
        tpt.fillrect(e - w, f - x, w * 2, x * 2, 255, 0, 0, 100)
    end
end

event.register(event.keypress, g)
event.register(event.mousedown, n)
event.register(event.mousemove, r)
event.register(event.mouseup, u)
event.register(event.tick, z)

Or you can start the brush at just 1 pixel by one pixel, then hold shift or ctrl will scrolling up with your mouse wheel to only extend the brush's length or width respectively.  With this method, you can draw entire lines either horizontal or vertical with just the click of a mouse, instead of having to hold and drag the mouse.

@AlexFinn (View Post)

To turn it off, simply press Shift again; to completely disable the script, you need to delete it.

@AlexFinn (View Post)

 You can use alt (or option on mac) to make a perfectly straight line.