fix https://github.com/sagemathinc/smc/issues/1322 -- Changing tabs between PDFs doesn't hide the previous one
fix https://github.com/sagemathinc/smc/issues/1322 -- Changing tabs between PDFs doesn't hide the previous one

DrXyzzy/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc