chat: apply Blizzard per-window transparency to docked backdrops

The native chat background transparency slider drives FCF_SetWindowAlpha,
which only touches the ChatFrame*Background textures pfUI hides on docked
frames, so the visible pfUI backdrop never responded to it.

Mirror the selected window's stored alpha (GetChatWindowInfo) onto the pfUI
backdrop and refresh it from RefreshChat, FCF_SetWindowAlpha and
FCF_SelectDockFrame so the native slider drives it live and persists across
/reload. Default the SetupPositions window alpha to 0.8 to keep the historical
look, and migrate existing installs whose windows still carry the old hard-0
alpha so their chat background is not suddenly transparent.

Fixes #48

modules/chat.lua

@@ -296,6 +296,54 @@ pfUI:RegisterModule("chat", function ()
     end
   end
 
+  -- [ Chat Background Alpha ]
+  -- Blizzard's per-window transparency slider drives FCF_SetWindowAlpha, which only
+  -- touches the ChatFrame*Background textures that pfUI hides on docked frames. Mirror
+  -- the window's stored alpha onto the visible pfUI backdrop so the native slider
+  -- controls it directly. The slider is authoritative for the panel's alpha.
+  local function ApplyChatBackgroundAlpha(panel, frame)
+    if not (panel and panel.backdrop and frame) then return end
+    local _, _, _, _, _, alpha = GetChatWindowInfo(frame:GetID())
+    alpha = tonumber(alpha)
+    if not alpha then return end
+    local r, g, b = panel.backdrop:GetBackdropColor()
+    panel.backdrop:SetBackdropColor(r, g, b, alpha)
+  end
+
+  function pfUI.chat:RefreshBackgroundAlpha()
+    -- left panel follows the currently selected docked tab
+    local selected = SELECTED_CHAT_FRAME
+    if not (selected and selected:GetParent() == pfUI.chat.left) then
+      selected = ChatFrame1
+    end
+    ApplyChatBackgroundAlpha(pfUI.chat.left, selected)
+
+    if C.chat.right.enable == "1" then
+      ApplyChatBackgroundAlpha(pfUI.chat.right, ChatFrame3)
+    end
+  end
+
+  function pfUI.chat:MigrateBackgroundAlpha()
+    -- Runs once per character. The previous SetupPositions default stored a hard 0 window
+    -- alpha, which now renders the pfUI backdrop fully transparent. Restore the historical
+    -- 0.8 look for any pfUI-managed window still sitting at 0. Only existing installs that
+    -- already ran chat setup carry that legacy 0; fresh installs get 0.8 from SetupPositions.
+    if pfUI_init.chatbgalpha then return end
+    if not pfUI_init["chat_position"] then return end
+    pfUI_init.chatbgalpha = true
+
+    local frames = { ChatFrame1, ChatFrame2 }
+    if C.chat.right.enable == "1" then table.insert(frames, ChatFrame3) end
+
+    for _, frame in ipairs(frames) do
+      local _, _, _, _, _, alpha = GetChatWindowInfo(frame:GetID())
+      alpha = tonumber(alpha)
+      if alpha and alpha <= 0 then
+        FCF_SetWindowAlpha(frame, 0.8)
+      end
+    end
+  end
+
   function pfUI.chat:RefreshChat()
     local panelheight = C.global.font_size*1.5 + default_border*2 + 2
 
@@ -447,9 +495,13 @@ pfUI:RegisterModule("chat", function ()
     for index, value in pairs(DOCKED_CHAT_FRAMES) do
       FCF_UpdateButtonSide(value)
     end
+
+    pfUI.chat:RefreshBackgroundAlpha()
   end
 
   hooksecurefunc("FCF_SaveDock", pfUI.chat.RefreshChat)
+  hooksecurefunc("FCF_SetWindowAlpha", function() pfUI.chat:RefreshBackgroundAlpha() end)
+  hooksecurefunc("FCF_SelectDockFrame", function() pfUI.chat:RefreshBackgroundAlpha() end)
 
   if C.chat.global.tabmouse == "1" then
     pfUI.chat.mouseovertab = CreateFrame("Frame")
@@ -500,7 +552,7 @@ pfUI:RegisterModule("chat", function ()
     FCF_SetLocked(ChatFrame1, 1)
     FCF_SetWindowName(ChatFrame1, GENERAL)
     FCF_SetWindowColor(ChatFrame1, 0, 0, 0)
-    FCF_SetWindowAlpha(ChatFrame1, 0)
+    FCF_SetWindowAlpha(ChatFrame1, 0.8)
     FCF_SetChatWindowFontSize(ChatFrame1, 12)
     ChatFrame1:SetUserPlaced(1)
 
@@ -508,7 +560,7 @@ pfUI:RegisterModule("chat", function ()
     FCF_SetLocked(ChatFrame2, 1)
     FCF_SetWindowName(ChatFrame2, COMBAT_LOG)
     FCF_SetWindowColor(ChatFrame2, 0, 0, 0)
-    FCF_SetWindowAlpha(ChatFrame2, 0)
+    FCF_SetWindowAlpha(ChatFrame2, 0.8)
     FCF_SetChatWindowFontSize(ChatFrame2, 12)
     ChatFrame2:SetUserPlaced(1)
 
@@ -518,7 +570,7 @@ pfUI:RegisterModule("chat", function ()
       FCF_SetLocked(ChatFrame3, 1)
       FCF_SetWindowName(ChatFrame3, T["Loot & Spam"])
       FCF_SetWindowColor(ChatFrame3, 0, 0, 0)
-      FCF_SetWindowAlpha(ChatFrame3, 0)
+      FCF_SetWindowAlpha(ChatFrame3, 0.8)
       FCF_SetChatWindowFontSize(ChatFrame3, 12)
       FCF_UnDockFrame(ChatFrame3)
       FCF_SetTabPosition(ChatFrame3, 0)
@@ -566,6 +618,9 @@ pfUI:RegisterModule("chat", function ()
   end
 
   pfUI.chat:SetScript("OnEvent", function()
+    -- restore legacy chat windows stuck at 0 alpha before anything reads it
+    pfUI.chat:MigrateBackgroundAlpha()
+
     -- set the default chat
     FCF_SelectDockFrame(SELECTED_CHAT_FRAME)