/Tukui/modules/blizzard/quests.lua

local T, C, L = unpack(select(2, ...)) -- Import: T - functions, constants, variables; C - config; L - locales

-- add quest level into Quest list frame.
if (IsAddOnLoaded("QuestGuru")) then return end

local function questlevel()
	local buttons = QuestLogScrollFrame.buttons
	local numButtons = #buttons
	local scrollOffset = HybridScrollFrame_GetOffset(QuestLogScrollFrame)
	local numEntries, numQuests = GetNumQuestLogEntries()
	
	for i = 1, numButtons do
		local questIndex = i + scrollOffset
		local questLogTitle = buttons[i]
		if questIndex <= numEntries then
			local title, level, questTag, suggestedGroup, isHeader, isCollapsed, isComplete, isDaily = GetQuestLogTitle(questIndex)
			if not isHeader then
				questLogTitle:SetText("[" .. level .. "] " .. title)
				QuestLogTitleButton_Resize(questLogTitle)
			end
		end
	end
end
hooksecurefunc("QuestLog_Update", questlevel)
QuestLogScrollFrameScrollBar:HookScript("OnValueChanged", questlevel)