UIManager:discardEvents(): use accurate "now"

Instead of UIManager possibly staled _now, which could have
no effect after a long ReaderRolling re-rendering.
Also lower the delay after such ReaderRolling re-rendering,
so we can change settings quicker when comparing renderings.

frontend/apps/reader/modules/readerrolling.lua

@@ -884,7 +884,9 @@ function ReaderRolling:onUpdatePos()
     self:updatePos()
 
     Device:setIgnoreInput(false) -- Allow processing of events (on Android).
-    UIManager:discardEvents(true) -- Discard events, which might have occured (double tap).
+    UIManager:discardEvents(0.2) -- Discard events, which might have occurred (double tap).
+    -- We can use a smaller duration than the default (quite large to avoid accidental dismissals),
+    -- to allow for quicker setting changes and rendering comparisons.
 end
 
 function ReaderRolling:updatePos()

frontend/ui/uimanager.lua

@@ -1081,7 +1081,7 @@ function UIManager:discardEvents(set_or_seconds)
     else -- we expect a number
         delay = TimeVal:new{ sec = set_or_seconds, usec = 0 }
     end
-    self._discard_events_till = self._now + delay
+    self._discard_events_till = TimeVal:now() + delay
 end
 
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