The box-shadow causes an image to be very blurry on Webkit browsers
(experienced: Konqueror, Safari; heard of: Chrome) when scaled up
if the initial size of the image is much smaller than the actual size
(e.g. when viewing a very long picture).

The shadow is not really obvious anyway.

The pointerup strategy is unsuccessful, as some other overlays
(Firefox's Inspect Element) will pass down pointerup events.

Now swiping will correctly change the current media, and with a good
preview. Clicking without swiping closes the overlay.

Originally the media viewer would think every touch is a swipe (one-finger
touch event), so we would encounter the case where a two-finger scale event
would incorrectly change the current media. This is now fixed.

linear => linear (now default)
simple_tree => tree / conversationTreeAdvanced=false
tree => tree / conversationTreeAdvanced=true

Ref: tree-threading

$refs is not a reliable way to deal with child components under
tree threading as it is not reactive, but the children may change at
any time. The only good way seems to be making these states aggregated on
the conversation component.

Ref: tree-threading