v2.7.12-lpc2024: html5: fix persistent matrix chat

v2.7.12 with LPC changes

v2.7.1-lpc2023: 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 <[email protected]>