Set "tabindex" to the container when it is scrollable
Set "tabindex" to the container when it is scrollable

Firefox automatically includes scrollable elements in the sequential
keyboard navigation, but it needs to be explicitly done for Chromium.

Moreover, setting "tabindex" is also needed in Chromium to be able to
scroll by keeping a key pressed after focusing on the scrollable element
by clicking on one of its children; in Firefox the event keeps being
processed and the scrolling goes on until the key is released, but in
Chromium, when "tabindex" is not set, the scrolling stops as soon as the
child element is removed (which in the case of the virtual list happens
when the element goes out of view).

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed