I use an old laptop hooked up to an old monitor and old loudspeakers. The only new component is a wireless keyboard with a trackpad. Indeed, can recommend. Kodi is nice if you don't want it to look and work too much like a PC.

In the new inertial system the block was moving to the left before dropping and was sped up to exactly standstill.

Not relative to itself though, which is the chosen frame of reference. My guess is the energy went into the ramp and whatever is connected to it (e.g. Earth). Now off to the comments to see if I'm right.

The second half of the video is a bit misleading, with the break in movement going downhill. But first off, there is no such thing as movement of the block relative to the block. Both systems (camera A, standing still relative to the hill and B moving at speed beta to the right) are inertial, so energy is conserved. In system B, the block is moving to the left at speed -v, and is accelerated to the right in such a way that it comes to a standstill relative to B. Kinetic energy itself isn't Loretnz invariant, but conservation of energy holds in every inerial system.