Window switching behaviour has changed
This morning I found out the window switching key now does a minimise/maximise thing rather than switching windows with one keypress.
You can still ALT+TAB to switch maximised windows, but the new feature allows you to have 2 pages side by side.
Details here:  louisgray.com: ChromeOS Now Supports Multiple Displayed Windows
I'm not that enthused about the new way of working
The Panelize extension worked well for me, and I liked the simplicity of the single keypress to toggle windows.