Another bug-fix update (five build numbers over last week's effort), but we have a new ('feature2', think 'RS2+') Fast ring build for Windows 10 Mobile. As before, the Fast Insiders ring for Windows 10 Mobile is becoming less the 'unstable, early access' ring and more the 'more bugs fixed than any other' ring. There's clearly still effort going into keeping the core of Windows 10 Mobile up to date, in parallel with the internals of the Desktop SKU, and fixing reported issues. There's also a new feature in terms of support for Continuum on portrait monitors.

From the Windows blog:

...We are also releasing Windows 10 Mobile Insider Preview Build 15235 to Insiders in the Fast ring. 

Switch to portrait mode orientation in Continuum

Are you an Windows Insider who is loving the convenience of Continuum by carrying your PC in your pocket? We have heard your VERY PASSIONATE feedback and just made it better. We’re excited to introduce portrait mode support for Continuum starting with this build of Windows 10 Mobile.

Without portrait mode support, the landscape rendering of Continuum from your phone on a portrait oriented monitor would look like this:

Now, it will look like this:

To enable this,  go to Settings > System > Display on your device and change the display orientation to “Portrait” and press apply.

If your phone is connected to a monitor via cable, your phone will ask you to disconnect and reconnect the cable to the external display and press ok to change the orientation to portrait. If your phone is connected to a monitor via Wi-Fi, your phone will ask you to disconnect and reconnect to your wireless display. Once this is done, your phone will remember the orientation and you will not have to repeat the setup process when connecting again to the same monitor.

General changes, improvements, and fixes for Mobile

• This build also includes all the improvements from KB4025342.
• We fixed an issue where downloading large apps could report out of disk space when there is sufficient space available.
• We fixed an issue where installing apps (particularly games) from the Windows Store would sometimes fail with “Error, see details”.
• We fixed an issue where a device could not be unlocked after removing/turning off Windows Hello.
• We fixed an issue where Cortana would not provide the region when saying “Your region is set to” and then “Is that right?”.
• We fixed a Turkish localization issue where the string “We need to fix your Microsoft account before you can use shared experiences …” was shown in English.

Known issues for Mobile

• There is a problem with the HP Elite X3 with wired docks where the portrait orientation setting is lost when the external display is disconnected and reconnected. A workaround for this is to reboot the phone after tapping the “OK” button instead of disconnecting and reconnecting. This workaround must be performed every time you connect to an external display you want to use with Continuum in portrait orientation.

So quite a gotcha for Elite x3 owners then, though only if they're intending to use the new feature above. And this is the Fast ring, remember, so I'm sure this will be ironed out before it hits production handsets.