Thank you so much!
Hardware: Dell Precision 7560, Dell Docking Station WD19DCS, two monitors with DisplayPort and HDMI
The solution was a bit more complicated in my case:
- It started at "Show only on 2".
- I selected "Extend these displays" but it was always switching back to display 2, without any message or the usual keep/revert dialog.
- I have tried many settings before I finally got it to work as expected. I think the relevant part was switching to "Show only on 1". I had to try this multiple times before it accepted that change (and you have to click "keep" fast enough, as usual).
- Then, switch to "Extend these displays".
Some additional observations, in hope that it might be useful to some who find this thread: The dock is rather unreliable and will often have a degraded connection to the laptop. This is usually (but not always) indicated by only one of the two lights being lit. Sometimes there is a message from Dell tools or BIOS (e.g. "multiple docks are not supported"). Sometimes you only notice by monitors not working as expected (e.g. higher resolutions not available or blurry image despite high resolution). We have these issues with many laptops and WD19 docking stations, so this is not a faulty laptop, dock or cable.
When the connection is degraded, both monitors remain on, i.e. the bug doesn't occur in that case. Windows still shows the vendor and model of both monitors but it seems that it cannot turn off the second monitor. This isn't useful, of course, because the degraded connection is not enough to run both monitors at a useful resolution.
The monitors have been bought at the same time and their DCI information are very similar. This might be part of the problem. Nevertheless, something must have changed in the last weeks because it was working fine for years.
I recently updated PowerToys and enabled the new "Power Display" feature but it can only control one of the monitors. The issue might be related to this but it wasn't fixed when I disabled Power Display.
