UIManager: Handle covers_fullscreen properly in close (#6828)

* When closing a widget, stop sending setDirty call for widgets lower on
the stack as soon as we hit a full-screen one.

This prevents inflated refresh regions when closing stuff on top of a
stack of multiple covers_fullscreen widgets (i.e., InfoMessages on top
of the Favorites page on top of the FM, for instance).

And, while we're there, also prevent getting infected by dithered
widgets when they're below a non dithered full-screen widget (the exact
same examples works, if the underlying FM page requires dithering).

frontend/ui/uimanager.lua

@@ -425,6 +425,8 @@ function UIManager:close(widget, refreshtype, refreshregion, refreshdither)
     -- make it disabled by default and check if any widget wants it disabled or enabled
     Input.disable_double_tap = true
     local requested_disable_double_tap = nil
+    local is_covered = false
+    local start_idx = 1
     -- then remove all references to that widget on stack and refresh
     for i = #self._window_stack, 1, -1 do
         if self._window_stack[i].widget == widget then
@@ -432,12 +434,23 @@ function UIManager:close(widget, refreshtype, refreshregion, refreshdither)
             table.remove(self._window_stack, i)
             dirty = true
         else
-            -- If anything else on the stack was dithered, honor the hint
-            if self._window_stack[i].widget.dithered then
+            -- If anything else on the stack not already hidden by (i.e., below) a fullscreen widget was dithered, honor the hint
+            if self._window_stack[i].widget.dithered and not is_covered then
                 refreshdither = true
                 logger.dbg("Lower widget", self._window_stack[i].widget.name or self._window_stack[i].widget.id or tostring(self._window_stack[i].widget), "was dithered, honoring the dithering hint")
             end
 
+            -- Remember the uppermost widget that covers the full screen, so we don't bother calling setDirty on hidden (i.e., lower) widgets in the following dirty loop.
+            -- _repaint already does that later on to skip the actual paintTo calls, so this ensures we limit the refresh queue to stuff that will actually get painted.
+            if not is_covered and self._window_stack[i].widget.covers_fullscreen then
+                is_covered = true
+                start_idx = i
+                logger.dbg("Lower widget", self._window_stack[i].widget.name or self._window_stack[i].widget.id or tostring(self._window_stack[i].widget), "covers the full screen")
+                if i > 1 then
+                    logger.dbg("not refreshing", i-1, "covered widget(s)")
+                end
+            end
+
             -- Set double tap to how the topmost specifying widget wants it
             if requested_disable_double_tap == nil and self._window_stack[i].widget.disable_double_tap ~= nil then
                 requested_disable_double_tap = self._window_stack[i].widget.disable_double_tap
@@ -448,8 +461,8 @@ function UIManager:close(widget, refreshtype, refreshregion, refreshdither)
         Input.disable_double_tap = requested_disable_double_tap
     end
     if dirty and not widget.invisible then
-        -- schedule remaining widgets to be painted
-        for i = 1, #self._window_stack do
+        -- schedule the remaining visible (i.e., uncovered) widgets to be painted
+        for i = start_idx, #self._window_stack do
             self:setDirty(self._window_stack[i].widget)
         end
         self:_refresh(refreshtype, refreshregion, refreshdither)