Bidirectional Scroll Frame

BidirectionalScrollTemplate.lua

local ScrollChild = CreateFrame("Frame")
ScrollChild:SetSize(750, 750)

local ScrollChildTexture = ScrollChild:CreateTexture()
ScrollChildTexture:SetPoint("CENTER")
ScrollChildTexture:SetTexture([[Interface\ICONS\TEMP]])

local ScrollFrame = CreateFrame("ScrollFrame")
ScrollFrame:SetPoint("CENTER")
ScrollFrame:SetSize(500, 500)
ScrollFrame:SetScrollChild(ScrollChild)

ScrollFrame.GetHorizontalPanExtent = function(self)
	return self.horizontalPanExtent
end

ScrollFrame.SetHorizontalPanExtent = function(self, panExtent)
	self.horizontalPanExtent = panExtent
end

ScrollFrame.GetVerticalPanExtent = function(self)
	return self.verticalPanExtent
end

ScrollFrame.SetVerticalPanExtent = function(self, panExtent)
	self.verticalPanExtent = panExtent
end

ScrollFrame:SetHorizontalPanExtent(30)
ScrollFrame:SetVerticalPanExtent(30)

local ScrollBarV = CreateFrame("EventFrame", nil, self, "WowTrimScrollBar")
ScrollBarV:SetHideIfUnscrollable(true)
ScrollBarV:SetHideTrackIfThumbExceedsTrack(true)
ScrollBarV:SetPoint("TOPLEFT", ScrollFrame, "TOPRIGHT")
ScrollBarV:SetPoint("BOTTOMLEFT", ScrollFrame, "BOTTOMRIGHT")
ScrollBarV:Show()

local ScrollBarH = CreateFrame("EventFrame", nil, self, "WowTrimHorizontalScrollBar")
ScrollBarH:SetHideIfUnscrollable(true)
ScrollBarH:SetHideTrackIfThumbExceedsTrack(true)
ScrollBarH:SetPoint("TOPLEFT", ScrollFrame, "BOTTOMLEFT")
ScrollBarH:SetPoint("TOPRIGHT", ScrollFrame, "BOTTOMRIGHT")
ScrollBarH:Show()

---
-- The below is largely just adapted from InitScrollFrameWithScrollBar
-- but with additional scripts and tweaks to support bidirectional scrolling.
---

local function CalculateScrollPercentage(scrollOffset, scrollRange)
	local scrollPercentage = 0
	if scrollRange > 0 then
		scrollPercentage = scrollOffset / scrollRange
	end
	return scrollPercentage
end

local function CalculateVisibleExtentPercentage(scrollExtent, scrollRange)
	local visibleExtentPercentage = 0
	if scrollExtent > 0 then
		visibleExtentPercentage = scrollExtent / (scrollRange + scrollExtent)
	end
	return visibleExtentPercentage
end

local function CalculatePanExtentPercentage(panExtent, scrollRange)
	local panExtentPercentage = 0
	if scrollRange > 0 then
		panExtentPercentage = Saturate(panExtent / scrollRange)
	end
	return panExtentPercentage
end

local function OnHorizontalScroll(self, offset)
	local scrollPercentage = CalculateScrollPercentage(offset, ScrollFrame:GetHorizontalScrollRange())
	ScrollBarH:SetScrollPercentage(scrollPercentage, ScrollBoxConstants.NoScrollInterpolation)
end

local function OnVerticalScroll(self, offset)
	local scrollPercentage = CalculateScrollPercentage(offset, ScrollFrame:GetVerticalScrollRange())
	ScrollBarV:SetScrollPercentage(scrollPercentage, ScrollBoxConstants.NoScrollInterpolation)
end

local function OnScrollRangeChanged(self, hScrollRange, vScrollRange)
	OnHorizontalScroll(self, self:GetHorizontalScroll())
	OnVerticalScroll(self, self:GetVerticalScroll())

	ScrollBarH:SetVisibleExtentPercentage(CalculateVisibleExtentPercentage(self:GetWidth(), hScrollRange))
	ScrollBarH:SetPanExtentPercentage(CalculatePanExtentPercentage(self:GetHorizontalPanExtent(), hScrollRange))
	ScrollBarV:SetVisibleExtentPercentage(CalculateVisibleExtentPercentage(self:GetHeight(), vScrollRange))
	ScrollBarV:SetPanExtentPercentage(CalculatePanExtentPercentage(self:GetVerticalPanExtent(), vScrollRange))
end

local function OnHorizontalBarScroll(self, scrollPercentage)
	local scroll = scrollPercentage * ScrollFrame:GetHorizontalScrollRange()
	ScrollFrame:SetHorizontalScroll(scroll)
end

local function OnVerticalBarScroll(self, scrollPercentage)
	local scroll = scrollPercentage * ScrollFrame:GetVerticalScrollRange()
	ScrollFrame:SetVerticalScroll(scroll)
end

local function OnMouseWheel(_, value)
	ScrollBarV:ScrollStepInDirection(-value)
end

ScrollFrame:SetScript("OnHorizontalScroll", OnHorizontalScroll)
ScrollFrame:SetScript("OnVerticalScroll", OnVerticalScroll)
ScrollFrame:SetScript("OnScrollRangeChanged", OnScrollRangeChanged)
ScrollFrame:SetScript("OnMouseWheel", OnMouseWheel)
ScrollBarH:RegisterCallback("OnScroll", OnHorizontalBarScroll, ScrollFrame)
ScrollBarV:RegisterCallback("OnScroll", OnVerticalBarScroll, ScrollFrame)