Toggle the item comparison live with shift

The comparison read IsShiftKeyDown() only when the tooltip was set, so
pressing or releasing shift while hovering did nothing until the next
hover. Each hooked tooltip now remembers the item it shows, dropped on
OnTooltipCleared and OnHide, and MODIFIER_STATE_CHANGED shows or hides
its comparison right away, as on retail.

Merchant and auction comparisons show regardless of shift and never
store a link, so shift leaves them up.

modules/eqcompare.lua

@@ -86,6 +86,12 @@ pfUI:RegisterModule("eqcompare", function ()
     end
   end
 
+  local function HideCompareItem(self)
+    local shoppingTooltip1, shoppingTooltip2 = unpack(self.shoppingTooltips or { ShoppingTooltip1, ShoppingTooltip2 })
+    shoppingTooltip1:Hide()
+    shoppingTooltip2:Hide()
+  end
+
   local prevMerchant = ShoppingTooltip1.SetMerchantCompareItem
   local function SetMerchantCompareItem(self, index, compareItem)
     if compareItem == 1 then
@@ -139,9 +145,18 @@ pfUI:RegisterModule("eqcompare", function ()
     deferred = true
   end)
 
+  -- the item each hooked tooltip shows, for MODIFIER_STATE_CHANGED. Every Set*
+  -- clears the tooltip before it builds, so OnTooltipCleared drops the link
+  -- of the previous content before the hook stores the new one.
+  local links = {}
+  local function forget()
+    links[this] = nil
+  end
+
   local function makeHook(getter)
     return function(tooltip, arg1, arg2, arg3)
       local link = getter(arg1, arg2, arg3)
+      links[tooltip] = link
       if not deferred then
         return ShowCompareItem(tooltip, link)
       end
@@ -157,8 +172,29 @@ pfUI:RegisterModule("eqcompare", function ()
     for setter, getter in pairs(TooltipHooks) do
       hooksecurefunc(tooltip, setter, makeHook(getter))
     end
+    tooltip:HookScript("OnTooltipCleared", forget)
+    tooltip:HookScript("OnHide", forget)
   end
 
+  -- pressing or releasing shift over an item toggles its comparison at once.
+  -- Merchant and auction comparisons show regardless of shift and never set
+  -- a link, so they stay up.
+  pfUI.events:RegisterFrameEventAndCallback("MODIFIER_STATE_CHANGED", function(_, key)
+    if key ~= "LSHIFT" and key ~= "RSHIFT" then return end
+
+    local shift = IsShiftKeyDown()
+    for tooltip, link in pairs(links) do
+      -- a hidden tooltip got its item from a scan, not a hover
+      if tooltip:IsShown() then
+        if shift then
+          ShowCompareItem(tooltip, link, shift)
+        elseif C.tooltip.compare.showalways ~= "1" or C_Item.IsEquippedItem(link) then
+          HideCompareItem(tooltip)
+        end
+      end
+    end
+  end)
+
   HookTooltip(GameTooltip)
 
   pfUI.eqcompare.HookTooltip = HookTooltip