fix night mode setting

implement bezel cleaning with proper background color

frontend/apps/filemanager/filemanagermenu.lua

@@ -129,7 +129,7 @@ function FileManagerMenu:setUpdateItemTable()
         checked_func = function() return G_reader_settings:readSetting("night_mode") end,
         callback = function()
             local night_mode = G_reader_settings:readSetting("night_mode") or false
-            Screen.bb:invert()
+            Screen:toggleNightMode()
             G_reader_settings:saveSetting("night_mode", not night_mode)
         end
     })

frontend/apps/reader/modules/readermenu.lua

@@ -103,7 +103,7 @@ function ReaderMenu:setUpdateItemTable()
         checked_func = function() return G_reader_settings:readSetting("night_mode") end,
         callback = function()
             local night_mode = G_reader_settings:readSetting("night_mode") or false
-            Screen.bb:invert()
+            Screen:toggleNightMode()
             G_reader_settings:saveSetting("night_mode", not night_mode)
         end
     })

frontend/device/screen.lua

@@ -160,6 +160,15 @@ function Screen:setScreenMode(mode)
     end
 end
 
+function Screen:toggleNightMode()
+    self.bb:invert()
+    if self.viewport then
+        -- invert and blank out the full framebuffer when we are working on a viewport
+        self.fb.bb:invert()
+        self.fb.bb:fill(Blitbuffer.COLOR_WHITE)
+    end
+end
+
 function Screen:saveCurrentBB()
     if self.saved_bb then self.saved_bb:free() end
     self.saved_bb = self.bb:copy()

reader.lua

@@ -103,7 +103,7 @@ end
 local open_last = G_reader_settings:readSetting("open_last")
 -- night mode
 if G_reader_settings:readSetting("night_mode") then
-    Screen.bb:invert()
+    Screen:toggleNightMode()
 end
 
 -- restore kobo frontlight settings