UIManager: avoid painting widgets covered by a full screen widget (#3770)

Navigating the TOC, viewing a full screen image, browsing
reading stats... would call paintTo() on ReaderUI (so, asking
the engine to render the page) or FileManager (so, rendering cover
images) even though their content is hidden.
Widgets registering to UIManager have to explicitely states
they cover the full screen (UIManager can't know, parts of their
dimen may be transparent, e.g. if it is a CenterContainer).

frontend/apps/filemanager/filemanager.lua

@@ -775,6 +775,7 @@ function FileManager:showFiles(path, focused_file)
     restoreScreenMode()
     local file_manager = FileManager:new{
         dimen = Screen:getSize(),
+        covers_fullscreen = true, -- hint for UIManager:_repaint()
         root_path = path,
         focused_file = focused_file,
         onExit = function()

frontend/apps/filemanager/filemanagerhistory.lua

@@ -128,6 +128,7 @@ function FileManagerHistory:onShowHist()
         ui = self.ui,
         width = Screen:getWidth(),
         height = Screen:getHeight(),
+        covers_fullscreen = true, -- hint for UIManager:_repaint()
         is_borderless = true,
         is_popout = false,
         onMenuHold = self.onMenuHold,

frontend/apps/opdscatalog/opdscatalog.lua

@@ -61,6 +61,7 @@ function OPDSCatalog:showCatalog()
     logger.dbg("show OPDS catalog")
     UIManager:show(OPDSCatalog:new{
         dimen = Screen:getSize(),
+        covers_fullscreen = true, -- hint for UIManager:_repaint()
         onExit = function()
             --UIManager:quit()
         end

frontend/apps/reader/modules/readerbookmark.lua

@@ -235,6 +235,7 @@ function ReaderBookmark:onShowBookmark()
 
     self.bookmark_menu = CenterContainer:new{
         dimen = Screen:getSize(),
+        covers_fullscreen = true, -- hint for UIManager:_repaint()
         bm_menu,
     }
 

frontend/apps/reader/modules/readertoc.lua

@@ -335,6 +335,7 @@ function ReaderToc:onShowToc()
 
     local menu_container = CenterContainer:new{
         dimen = Screen:getSize(),
+        covers_fullscreen = true, -- hint for UIManager:_repaint()
         toc_menu,
     }
 

frontend/apps/reader/readerui.lua

@@ -467,6 +467,7 @@ function ReaderUI:doShowReader(file, provider)
     G_reader_settings:saveSetting("lastfile", file)
     local reader = ReaderUI:new{
         dimen = Screen:getSize(),
+        covers_fullscreen = true, -- hint for UIManager:_repaint()
         document = document,
     }
     UIManager:show(reader)

frontend/ui/screensaver.lua

@@ -141,6 +141,7 @@ function Screensaver:show(event, fallback_message)
         UIManager:close(self.left_msg)
         self.left_msg = nil
     end
+    local covers_fullscreen = true -- hint for UIManager:_repaint()
     local overlay_message
     local prefix = event and event.."_" or "" -- "", "poweroff_" or "reboot_"
     local screensaver_type = G_reader_settings:readSetting(prefix.."screensaver_type")
@@ -255,6 +256,7 @@ function Screensaver:show(event, fallback_message)
         local screensaver_message = G_reader_settings:readSetting(prefix.."screensaver_message")
         if not self:whiteBackground() then
             background = nil -- no background filling, let book text visible
+            covers_fullscreen = false
         end
         if screensaver_message == nil then
             screensaver_message = fallback_message or default_screensaver_message
@@ -275,6 +277,7 @@ function Screensaver:show(event, fallback_message)
         self.left_msg = ScreenSaverWidget:new{
             widget = widget,
             background = background,
+            covers_fullscreen = covers_fullscreen,
         }
         self.left_msg.modal = true
         -- refresh whole screen for other types

frontend/ui/uimanager.lua

@@ -621,11 +621,26 @@ function UIManager:_repaint()
     -- will trigger a refresh if set.
     local dirty = false
 
-    for _, widget in ipairs(self._window_stack) do
+    -- We don't need to call paintTo() on widgets that are under
+    -- a widget that covers the full screen
+    local start_idx = 1
+    for i = #self._window_stack, 1, -1 do
+        if self._window_stack[i].widget.covers_fullscreen then
+            start_idx = i
+            if i > 1 then
+                logger.dbg("not painting", i-1, "covered widget(s)")
+            end
+            break
+        end
+    end
+
+    for i = start_idx, #self._window_stack do
+        local widget = self._window_stack[i]
         -- paint if current widget or any widget underneath is dirty
         if dirty or self._dirty[widget.widget] then
             -- pass hint to widget that we got when setting widget dirty
             -- the widget can use this to decide which parts should be refreshed
+            logger.dbg("painting widget:", widget.widget.name or widget.widget.id or tostring(widget))
             widget.widget:paintTo(Screen.bb, widget.x, widget.y, self._dirty[widget.widget])
 
             -- and remove from list after painting

frontend/ui/widget/bookstatuswidget.lua

@@ -93,6 +93,7 @@ function BookStatusWidget:init()
     end
 
     local screen_size = Screen:getSize()
+    self.covers_fullscreen = true -- hint for UIManager:_repaint()
     self[1] = FrameContainer:new{
         width = screen_size.w,
         height = screen_size.h,

frontend/ui/widget/imageviewer.lua

@@ -100,6 +100,9 @@ function ImageViewer:init()
             Swipe = { GestureRange:new{ ges = "swipe", range = range } },
         }
     end
+    if self.fullscreen then
+        self.covers_fullscreen = true -- hint for UIManager:_repaint()
+    end
     self:update()
 end
 

frontend/ui/widget/keyvaluepage.lua

@@ -278,6 +278,9 @@ function KeyValuePage:init()
         w = self.width or Screen:getWidth(),
         h = self.height or Screen:getHeight(),
     }
+    if self.dimen.w == Screen:getWidth() and self.dimen.h == Screen:getHeight() then
+        self.covers_fullscreen = true -- hint for UIManager:_repaint()
+    end
 
     if Device:hasKeys() then
         self.key_events = {