html5: fix persistent matrix chat

using visibility: hidden still causes the element to do layout, which
squeeze other elements (like timer, polls, etc).  Fix this by changing
to display: none which doesn't do any layout at all.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

Author: jejb

bigbluebutton-html5/imports/ui/components/matrix/component.jsx

@@ -56,7 +56,7 @@ const Matrix = ({
     <Styled.Matrix
 	data-test="matrix"
 	isChrome={isChrome}
-	style={{ visibility: isVisible ? 'visible' : 'hidden', }}
+	style={ isVisible ? {} : {display: 'none'}}
      >
       <Header
         leftButtonProps={{