When wanting to cast a new tab or other source to a sink that currently has an active route, that route must be stopped before casting the new source. This isn't necessary with the legacy cast extension.
The general feature can be tested with these steps:
1. Enable Media Router with the media-router flag, either via command line or in chrome://flags.
2. Cast something to a device; let's consider casting a tab. Click "Cast..." from the browser overflow menu or the context menu and select a cast device.
3. Observe that once the casting starts, there is only a "stop" button shown in the dialog (i.e. no "cast" button is shown).
4. Go to another tab.
5. Open the Media Router dialog again (right click > "Cast...").
6. You should see a description of the previous cast action and there should now be a "cast" button and a "stop" button.
7. Click "cast".
8. Observe that the new tab is now being cast and there is no longer a cast button shown next to the "stop" button.
Comment 1 by bugdroid1@chromium.org
, Jun 2 2016