RFC: effects: Disable the morphing popups effect by default

The effect does not work very well for two main reasons:

Some clients will make a new popup rather than move an existing one, so whether it does anything is highly unpredictable as a user.

Popups can be of massively different sizes with different amounts of text. This means the text in the smaller popup gets resized which doesn't look like a natural in-between state of the two popups.