WowAce

Classic compatibility was apparently dropped in LibUIDropDownMenu-4.0 versus earlier versions (-2.0).

If you wish to restore compatibility, the following changes will get you part of the way there...

local function creatre_DropDownList(name, parent)

  -- leave everything up to the OnShow script...

  f:SetScript("OnShow", function(self)
    -- leave everything currently in the OnShow script...

    -- at the bottom of the OnShow script, insert the following
    if (not UIDropDownMenu_HandleGlobalMouseEvent) then
      self.hideTimer = self.hideTimer or C_Timer.NewTicker(L_UIDROPDOWNMENU_SHOW_TIME, function()
        if (GetMouseFocus() ~= self) then
          self:Hide();
        end
      end)
    end
  end)

  f:SetScript("OnHide", function(self)
    -- leave everything currently in the OnShow script...

    -- at the bottom of the OnHide script, insert the following
    if (self.hideTimer) then
      self.hideTimer:Cancel();
      self.hideTimer = nil;
    end
  end)

  return f
end

-- hooking UIDropDownMenu_HandleGlobalMouseEvent
do
  if lib and UIDropDownMenu_HandleGlobalMouseEvent then
    hooksecurefunc("UIDropDownMenu_HandleGlobalMouseEvent", function(button, event)
      lib:UIDropDownMenu_HandleGlobalMouseEvent(button, event)
    end)
  end
end

Hi DahkCeles, there was some problem with curseforge so my early release (v4.01) was never sent out. I did implemented some codes to handle the global mouse event, although not exactly the same way like your implementations (I didn't add those codes in OnShow / OnHide). Could you help to check if those are still suggested in the latest revision?

I don't actually use the Lib any more in my own addon, so I'll have to wait until one of the other addons updates to test for this specific issue.

Meanwhile, I glanced over the code in 4.03 here on curse and found the following change you might want to consider.  This will simplify the differences between Classic Era, Burning Crusade Classic and Shadowlands.

Line 136

	-- ColorSwatch
	local fcw
--	if WoWClassic then
--		fcw = CreateFrame("Button", name.."ColorSwatch", f)
--	else
--		fcw = CreateFrame("Button", name.."ColorSwatch", f, BackdropTemplateMixin and "ColorSwatchTemplate" or nil)
--	end
	fcw = CreateFrame("Button", name.."ColorSwatch", f, BackdropTemplateMixin and DropDownMenuButtonMixin and "BackdropTemplate,ColorSwatchTemplate" or BackdropTemplateMixin and "BackdropTemplate" or nil);
	fcw:SetPoint("RIGHT", f, -6, 0)
	fcw:Hide()
--	if WoWClassic then
	if not DropDownMenuButtonMixin then
		fcw:SetSize(16, 16)
		fcw.SwatchBg = fcw:CreateTexture(name.."ColorSwatchSwatchBg", "BACKGROUND")
		fcw.SwatchBg:SetVertexColor(1, 1, 1)
		fcw.SwatchBg:SetWidth(14)
		fcw.SwatchBg:SetHeight(14)
		fcw.SwatchBg:SetPoint("CENTER", fcw, 0, 0)
		local button1NormalTexture = fcw:CreateTexture(name.."ColorSwatchNormalTexture")
		button1NormalTexture:SetTexture("Interface\\ChatFrame\\ChatFrameColorSwatch")
		button1NormalTexture:SetAllPoints()
		fcw:SetNormalTexture(button1NormalTexture)
	end

 Implemented

I use the StartCounting / StopCounting mechanism which was on early version of WoW to implement the show/hide features. Looks to be working more smoothly.