MouseLeave not working when mouse moves fast? How to fix?

If you're really struggling with this, I made a very messy module (must be created as ModuleScript and placed on the client).

local Mouse = require(script.Parent.Module) -- Path to the ModuleScript
local button = script.Parent;

local ButtonEvent = Mouse:new(button);


-- Fires once mouse has left the button
ButtonEvent:MouseLeave(function()
    print'left'
end)

-- Fires once mouse has entered the button
ButtonEvent:MouseEnter(function()
    print'entered'
end)

-- Once you don't want to listen to it anymore
ButtonEvent:destroy();

Note that this binds itself to RenderStepped.

Module code:

local RunService = game:GetService("RunService")
local HttpService = game:GetService("HttpService")
repeat wait() until game.Players.LocalPlayer
local Player = game.Players.LocalPlayer
local mouse = Player:GetMouse()


function collides(gui1, gui2)
    local g1p, g1s = gui1.AbsolutePosition, gui1.AbsoluteSize;
    local g2p, g2s = gui2.AbsolutePosition, gui2.AbsoluteSize;
    return ((g1p.x < g2p.x + g2s.x and g1p.x + g1s.x > g2p.x) and (g1p.y > g2p.y and g1p.y < g2p.y + g2s.y)) 
    or ((g2p.x < g1p.x + g1s.x and g2p.x + g2s.x > g1p.x) and (g2p.y > g1p.y and g2p.y < g1p.y + g1s.y));
end;


local followGui = Instance.new("ScreenGui")
followGui.Parent = Player:WaitForChild("PlayerGui");
followGui.Name = "MouseFollowGui"
followGui.ResetOnSpawn = false
local followFrame = Instance.new("Frame");
followFrame.Size = UDim2.new(0,1,0,1)
followFrame.Parent = followGui

mouse.Move:Connect(function()
    followFrame.Position = UDim2.new(0, mouse.X, 0, mouse.Y)
end)


local mouseObj = {
    isActive = true,
    binded = nil,
    lastState = false,
    mouseEnters = {},
    mouseLeaves = {}
}

function fireCallbacks (tab, ...)
    for i,v in pairs(tab) do
        v(...)
    end
end

function mouseObj:destroy()
    self.isActive = false
    RunService:UnbindFromRenderStep(self.binded)
    return true
end

function mouseObj:new (obj)
    if not obj:IsA("GuiObject") then return error("Not a valid GuiObject") end

    local newMouse = {};
    setmetatable(newMouse, self)
    self.__index = self;

    self.binded = HttpService:GenerateGUID(false)

    RunService:BindToRenderStep(self.binded, Enum.RenderPriority.Last.Value, function()
        local doesCollide = collides(followFrame, obj)
        if self.lastState == false and doesCollide then
            self.lastState = true
            fireCallbacks(self.mouseEnters)
        elseif self.lastState == true and not doesCollide then
            self.lastState = false
            fireCallbacks(self.mouseLeaves)
        end
    end)

    return newMouse;
end

function mouseObj:MouseEnter(cb)
    if type(cb) ~= "function" then return error("Not a valid callback parameter") end
    table.insert(self.mouseEnters, cb)
end

function mouseObj:MouseLeave(cb)
    if type(cb) ~= "function" then return error("Not a valid callback parameter") end
    table.insert(self.mouseLeaves, cb)
end



return mouseObj;