The new rendering approach has been a bad experience for me from the start, and hasn't improved.
This is largely because my app is a little complex and makes a lot of use of auto-height, with elements within a gallery which size and resize according to the contents as some are hidden or shown.
I'm also seeing different behaviours across different browsers, and mobile. For example in Chrome my autoheight labels don't expand, they just show about 5 pixels worth of height - the tops of letters. The same is true when running the PowerApp on my phone using the client app, yet using Edge it renders as expected.
As well as the display issues there are also massive performance issues. Again, this might be due to the particulars of my apps - I know that simpler apps I've developed work better in the new rendering engine. But the app I'm worried about is slow in the new engine - any click that results in a UI change, and even scrolling, feels like wading through thick mud.
Conversely the classic engine runs the same app perfectly smoothly.
What are the timescales for removal of the classic engine?