Multiple monitors broken

[Erez]

I have a Lenovo P50 with Nvidia Optimus (Quadro M1000M), installed with Ubuntu 16.04 and the recommended Nvidia drivers (361.42).
I added two external monitors connected to DP sockets in the docking station. I would like to have display shift to those two monitors when the laptop is docked. One monitor has native resolution of 1680x1050 and the other is 1920x1080. The laptop native resolution is 1920x1080.

Expected result:
laptop monitor shut off, two external monitors running in native resolution, display shared on both monitors (not mirrored).

Process:
I configured the display using the "Displays" applet in the unity control center. However, pressing "Apply" when changing a specific display setting (such as resolution, location of screen relative to others, turning on/off), results in a long period of darkness, usually followed by display falling back to mirroring of all 3 monitors, using a similar-size screen (either low on all or high on all with the low-res monitor using panning). The applet remains frozen for some time, with the dialog "Is the display ok" open but unresponsive. Later I realized this is because there is another dialog of "Cannot set screen CRTCXXX" hidden under the two above windows, which needs to by ok-ed first.

Extra:
I also tried setting the display manually with 'xrandr', which lead to a similar result as above - with the "cannot set screen" dialog, and falling back to mirroring. This convinced me that the problem is probably with the nvidia driver and not the higher plumbing.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: nvidia-361 361.42-0ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-34.53-generic 4.4.15
Uname: Linux 4.4.0-34-generic x86_64
NonfreeKernelModules: symap_custom_dkms_x86_64 nvidia_uvm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Aug 22 19:14:55 2016
InstallationDate: Installed on 2016-07-17 (36 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: nvidia-graphics-drivers-361
UpgradeStatus: No upgrade log present (probably fresh install)

[Erez (erez-hadad)]

Update: I thought it might be a permission problem, so I tried configuring my monitors manually using "sudo xrandr...". Same result - error dialog and fallback to mirroring.

I attach a more accurate photo of the error dialog that pops up at every failure - sometimes for more than one monitor, with different identifiers of "CRTCXXX".

[Erez (erez-hadad)]

Another attempt: I created a second dummy user, and logged in as that user. All screens are now set ok, each in the native resolution. However, any attempt (via "displays") to slightly change layout, order, resolution - results in the same issues of failure and fallback to mirroring. The configuration seems extremely fragile.

[Erez (erez-hadad)]

I'm attaching a log of LightDM showing the repeated segmentation faults at the Nvidia driver.
I believe those crashes correlate with the display changes happening when I log in or try to reconfigure the display, resulting in the fallback to mirroring I documented before.

Can anyone please pay attention to this issue?

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[thijs van severen (thijsvanseveren)]

same here :

my setup :

- Nvidia Driver (nvidia-smi) : 367.57
- Lenovo P50 vid cards (lspci | grep VGA)
    00:02.0 VGA compatible controller: Intel Corporation Skylake Integrated Graphics (rev 06)
    01:00.0 VGA compatible controller: NVIDIA Corporation GM107GLM [Quadro M1000M] (rev a2)
- bios version (sudo dmidecode --type 0 --type 13) : N1EET52W (1.26 )
- kernel version (uname -a) : Linux ThinkPad-P50 4.6.7-040607-generic #201608160432 SMP Tue Aug 16 08:35:05 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
- display manager (lightdm -v) : lightdm 1.18.3
- unity version (unity --version) : unity 7.4.0

When i'm running unity it does not work

i installed XFCE4 since a lot of people seem to report that this is the fix and ... tadaaa ... it works

not sure what display manager XFCE uses, but it sure looks like lightdm or unity is what is causing these issues
i'm ok with xfce, but i have grown to like unity so i really, really hope this gets fixed asap :-(

[Tad Morgan (tadm)]

same here as well, my configuration is the same as thijsvanseveren except:

kernel version (uname -a): 4.4.0-59-generic #80-Ubuntu SMP Fri Jan 6 17:47:47 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Have not tried xfce since I'd rather stay with unity.

[Marcin Garski (garski.marcin)]

same at my P50. Is there any solution without installing xfce.

dmidecode info:
# dmidecode 3.0
 Vendor: LENOVO
 Version: N1EET57W (1.30 )
 BIOS Revision: 1.30
 Firmware Revision: 1.15
 Product Name: 20EN0004PB
 SKU Number: LENOVO_MT_20EN_BU_Think_FM_ThinkPad P50
 Product Name: 20EN0004PB

In BIOS I changed Display: Hybrid Graphics to Disabled just to use Nvidia GPU, is it correct?
If it helps if I updated bios to the latest 1.37?

Thx. in advance.

Btw. What is the status of this issue? Is there any progress?

Gary

[Jose Arrarte (jarrarte)]

I have a similar situation with my P50, the dock station and a 1440x900 VGA monitor (ViewSonic VA1903wb). When docking the notebook or connecting the monitor after the notebook is docked results in a black screen + flashes loop that never returns (well, it looped for at least ~5 minutes before I unplugged the external monitor).

I'm using NVidia drivers v367.

I tried different kernels (stock 4.4, 4.7 and I'm on 4.8 now). 'uname -a' reports:
Linux jarrarte-nb 4.8.0-36-generic #36~16.04.1-Ubuntu SMP Sun Feb 5 09:39:57 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

[ritterkeks (ritterkeks)]

to add my two cents: the situation is totally different for users which plug in their displays directly into the notebook's ports and users trying to use the outputs of one of the docking stations.
According to my prior research for this problem this is caused by Lenovo not using simply pass-through connectors via the docking port anymore, but instead they are now using a single DisplayPort MST (MultiStreamTransort) connection to the dock, where a DisplayPort MST Hub splits the signals to the various outputs (converting them also in case of VGA out).

MST support seems to work fine only for some (4K) displays which report 2 panels with half the resolution each via DP, but not for those hubs atm, sadly. Maybe it works with the newest kernel and (as of yet unreleased) nouveau driver.
So the issues with directly plugging in displays should not be confused.

Other than that:
NVidia Driver 378 (currently newest release)
Linux 4.10.1 (mainline)
Bios revision: 1.38

But even with all the newest releases, still no improvement versus 4.4 kernel and recommended nvidia version...
Although if I am very lucky, I may be able to boot with two external displays attached which may sort of work, but then the desktop on the rightmost display is always panning for some reason and hotplugging anything (removing notebook from dock or simply unplugging a display) leads to the issue described by OP again. So my dock is for now simply being used as a VERY expensive USB (and power) hub ;)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Jose Arrarte (jarrarte)]

As mentioned in #10, it seems that the issue is related to the dock.

When connecting a Dell P2417H monitor to the dock using the DisplayPort connection, I can replicate this issue. But if I connect the same monitor to the HDMI port on the P50 (instead of the dock), it's detected and I can use it as an external display (it always maximizes to the same monitor, but that's a different problem).

I don't have a Mini DisplayPort cable/adapter to test the monitor connected directly to the notebook's DisplayPort.

[Erez (erez-hadad)]

I have developed the following hack as a workaround:
1. After configuring all the displays to work the way I want, I back up the ~/.config/monitors.xml to another location, naming the copy "monitors.xml.good"
2. When I put the laptop to the docking station and display gets messed up or falls back to mirroring, I do the following recovery procedure:
2.1 Unlock/Log in (in the messed-up display), find any error dialogs of "Cannot set screen.." and click on the ok button. This allows the display to later reconfigure when recover the configuration.
2.2 Lock the screen (Ctrl+Alt+L). This is important - I find that display reconfiguration works much better when the screen is locked.
2.3 Switch to a text console (Ctrl+Alt+F1)
2.4 Login (text)
2.5 Copy the monitors.xml.good backup over the existing ~/.config/monitors.xml
2.6 Turn off the secondary monitors (by pressing their power button)
2.7 Switch back to graphical login (Ctrl+Atl+F7)
2.8 Wait for the screen setting to settle down
2.9 Turn on the secondary monitors
2.10 Unlock screen (username/password)

That's it. At this point, the display is reset to the good configuration.
This hack is a bit cumbersome, since I move with my laptop to different rooms with different external screens and projectors, so in fact I maintain an accumulated monitors.xml containing all the working configurations for these screens. Still, better than frustration.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Raz (razzz)]

I'm having similar issues with the workstation dock, so I figured I'd add my details as well, in case it helps with debugging.

First of all, my setup:

OS: Ubuntu Gnome 16.04 LTS
NVIDIA driver version: 375.39
BIOS version: N1EET64W (1.37)
BIOS display setting: Discrete
Video card: 01:00.0 VGA compatible controller: NVIDIA Corporation GM107GLM [Quadro M1000M] (rev a2)
Kernel version: 4.8.0-41-generic
Display manager version: gdm3 3.20.1
GNOME shell version: GNOME Shell 3.20.4

If I plug my external monitor directly into the laptop, I have no issues whatsoever. As soon as I try running with one plugged into the dock, the display just constantly switches back and forth between the laptop screen by itself and both screens. Sometimes, I can boot fine with a display plugged into the dock, but as soon as anything changes, like the displays idling or changing a display setting, it starts flickering again.

I've attached my /var/log/Xorg.0.log, which confirms this behavior.

For this log dump, I did the following:
1. Powered on the laptop in the dock, with a single monitor plugged into the dock via HDMI.
2. Everything comes up fine and both monitors are working.
3. In this case, I just locked the screen, which causes the monitors to dim and turn off.
4. As soon as that happened, the flickering started up again
5. Unplugged the monitor from the dock, so I could use the computer again

The log file shows that, for some reason, the module is constantly "Setting mode" between the two different configurations.

[Vít Zikmund (tlwspm)]

I have the same setup as @ritterkeks in comment #10, and the same issues.

The P50 dock and it's DisplayPort hub is apparently the problem. The mini-DP connector on the laptop itself is working much better (I'm also seeing some OpenGL bugs there on my Lenovo P27 4K monitor (had to enable triple buffering), but that's apparently something else.

When I connect the monitor to either dock's DP ports the monitor's power led goes green like it's receiveng some signal, bit then goes amber into a power safe more. In the meantime the builtin monitor does pretty much the same, except it also shows some picture for a fraction of a second. This repeats sometimes forever (till I time out and pull the monitor's DP cable from the dock), or sometimes the xserver gives up and falls back to the builtin screen itself.
It happenede that it was sometimes successful and I saw a proper picture on both screens, but that was far from stable/usable. After a bit of desperate searching I've fould that the dock itself has an upgradable firmware. The current latest version is 2.33. After jumping through hoops of installing windows (thanks a ton, Lenovo) I've managed to flash the dock. To my surprise, the monitor adoption got worse and since then, I can't ever see any picture on the 4K monitor via the dock at all (but compared to the previous unusable state it's really no big deal). With Windows it works fine, though, so I'm sure the firmware upgrade didn't really brick it.

For those who want to try flashing the dock too 3:), you can find the binary here: https://support.lenovo.com/us/en/accessories/B697B003-94C2-418E-914F-23C201052DB1
Note the flashing utility is itself a "brilliant masterpiece" and most of the time it does nothing after you run it. At least apparently, but it does produce an "update.txt" file in the CWD, which you can examine for a status message. Also be sure to connect something to the dock when flashing it, as otherwise the utility shows it can't connect and bails out. So much facepalm.

[Bruno Katzengold (katzengold)]

I also still have these problems.
Only solution for me as yet:
using an adapter to connect my vga-monitor to the minidisplayport directly on the p50.

so for me, the dock is also just a usbhub at the moment (sadly)

[Bruno Katzengold (katzengold)]

https://forums.lenovo.com/t5/ThinkPad-P-and-W-Series-Mobile/Lenovo-P50-second-monitor-issue-in-Ubuntu-16-04/m-p/3620263#M66512

[Raz (razzz)]

After doing a bit more investigation, I found that the older Lenovo W541's dock has a similar MST chip inside (it populates DFP-1.1 and DFP-1.2, as opposed to our DFP-3.1 and DFP-3.2). However, it doesn't have the same issues that we have with the P50. One of my co-workers has a similar set-up as me with a W541, so if there's any information from there that would be useful for comparison, I can try to provide it.

I've also tried XFCE as others have said before, and it was able to use all monitors fine, with the one weird quirk that it seemed like it detected the new monitors several times when I docked while logged in. It's just a bit too minimal for me, and I've gotten used to the nicities that GNOME or Unity provide.

[Jory Morrison (jorymorrison)]

I am experiencing the same issue on Fedora 25 running gnome.

Any sort of change to the default monitor configuration results in mirroring between the two displays. Namely, I am trying to rotate my secondary display or change the resolution of the secondary display.

[fabio.hipolito (fabio-hipolito)]

Similar problems with the particularly annoying flashing desktop. With
nvidia-graphics-drivers-378
from
https://launchpad.net/~graphics-drivers/+archive/ubuntu/ppa
I can dock to one external monitor using display-port to d-dvi connection. Multiple external monitors remain always duplicated.

[fabio.hipolito (fabio-hipolito)]

Same problem is present in Ubuntu 17.04

[kristian (berserk)]

I also still got the same problem in Ubuntu 17.04

But is this really (only) nvidia related?
It is working for me with 3 monitors (laptop + 2 external via docking) in Xubuntu (XFCE) + nvidia 367.57

[fabio.hipolito (fabio-hipolito)]

It appears that it is possible to stop the flickering by forcing the restart of X with Crtl+Alt+Bksp, this is by no way a decent workaround but eventually it stabilizes and I get three independent screens working properly.

My hardware is a Lenovo Thinkpad P50 with nvidia Quadro M1000M connected to a Lenovo Thinkpad Dock, which in turn connects to two Lenovo ThinkVision via display port to DVI-D connecters.

I am running Ubuntu Gnome 17.04 with nvidia driver 375.39-0ubuntu5.

[Michael Grivas (mgrivas)]

Some of the aforementioned effects disappeared for me with the following:
1. Log out
2. Get in to console (alt+ctrl+f2 - for example) and login as user.
3. Delete .nvidia-settings-rc and the whole .nv/
3. Back to graphical (alt+ctrl+f7) - NO login yet.
4. Choose Gnome Classic or Gnome (default) (NOT ubuntu)
5. Log in.

Setup: P50, docking station, ubuntu 16.10 updated to 4.8.0-51, with nvidia 381.09.

To me, it seems that the problem is in Unity trying to set the nvidia some way.

[Mike Røntved (mikeyr)]

I have the same issue on Ubuntu 17.04 with Nvidia 375.66.
Using two 1920x1200 display connected to Thinkpad Docking Station 230W with Displayport.
Then I'm using the default monitor in Thinkpad P51, 1920x1080p.

In Ubuntu I have hard time getting all three displays to work, once they work I can't adjust any display settings (e.g. moving one monitor to the right side of another), without everything chrashing and I have to restart lightdm in terminal view.

However I'm capable of getting triple monitor setup correct in i3wm, and then using arandr as setup, but my laptop monitor (which is not set as primary), kinda works like an extension to my primary monitor, the workspaces works fine on all three monitors but when I'm opening e.g. rofi or dmenu on my laptop it stretches over all screens (dmenu doesn't even open), but when I go to one of the two connected monitors it works just fine.

If you need any files please tell.

[rengaw83 (rengaw83)]

Same problem here on Lenovo Thinkpad P71 with Nvidia Quadro P3000.
After connecting one ore more monitors (whether over hdmi, dvi or dp) via the Docking Station, all displays will turn black and flicker.
The only way to get the system back is to take the P71 out of the dock.
I have testet diffrent settings (additional bumblebee, prime, ...) and diffrent desktops (Ubuntu, Gnome, Cinnamon) nothing helps.
After many restarts and plug in and unplug the monitors again, the displays will be identified right and showing the desktop. But after changing the nvidia settings (e.g. monitor position) or readd the P71 to the dock, all monitors will turn black and flicker.

I use Ubuntu 16.04.3 LTS and the currently latest proprietary nvidia driver 384.59.

Is there a way to get it to run?

[Erez (erez-hadad)]

See my post from March 22 for a possible workaround.
Notes:
1. YMMV. The changes in kernel and nvidia driver sometimes cause the desktop to crash - not system hang, just need to login again and restart the desktop.
2. Possible improvement: try docking your laptop when the external monitors are turns off (but still connected), and then turn them on. It helps me sometimes restore the desktop quicker.

-- Erez

[Erez (erez-hadad)]

NVidia 384.90 - latest update - things seem to be taking a turn for the worse.
Now, I'm using just a single external monitor. Every time the screen sleeps after being idle for some time, it cannot be turned back on. As simple as that. No matter what I press. Not even by going back to console (ctrl+alt+f1) and overwriting monitors.xml. The only solution is to disconnect the external monitor. So I'm back at a single display - the laptop's.

As an even broader problem, trying to change display configuration using xrandr seems to be at its all-time worst. Almost any change will cause the display to crash back to blinking.

After being a long time Ubuntu user (6+ years), I can safely say that Ubuntu 16.04 on Lenovo P50 is my worst user experience in terms of stability - especially that of the display driver, when using my multi-monitor workstation. I have never had to tweak, reset configuration, kill my session or simply reboot so many times because of display issues. And its even worse since Lenovo P50 is "certified" in Ubuntu.

[fabio.hipolito (fabio-hipolito)]

My experience with version 384.90 remains the same, it is usable, as long as:
1. plug the laptop and the screens to the dock before booting the machine
2. do not under any circumstance hibernate or use screen lock

If you get the blinking screen, drop to a shell, say "Crtl+Shift+F3", and reboot the machine or force a X restart with "Ctrl+Alt+Bksp" (if you have activated this shortcut)

As far as I remember, only the laptop is certified, not the combination laptop+dock.
I share your frustration, but this is not a problem of ubuntu on its own, its a problem with the nvidia linux driver

[Erez (erez-hadad)]

Thanks for the cheerup, Fabio. I really wish to avoid reboots and X restarts since they kill my work sessions and disrupt my flow.
Does anyone know if this catastrophe is going to improve in 18.04 or Wayland?

[Vít Zikmund (tlwspm)]

Erez, try the laptop's mini DisplayPort. I use it since commenting on this bug before, suspend works, switching resolution rather works, and it's not that of a hassle to connect/disconnect it when you need to take the laptop out of the dock. Yes it defies part od the dock's purpose, but your life will be much less frustrating, trust me ;)

[Erez (erez-hadad)]

Guys, my use-case is simple. I use my laptop moving around and travelling, and come back to dock it with two monitors connected to the docking station. That used to work great with 14.04 on an older laptop but ever since I switched to the P50 with 16.04 it has been going gradually worse. Zikmund, I appreciate your good intentions, but I don't want to repeatedly plug monitors in and out of my laptop. I just want to dock it and have the displays set up and working automatically. Windows and MacOS folks do it all the time with ease.

[chessplayer (chessplayer)]

Hi guys,
I am joining this community of frustrated dock users. I am running Linux Mint Sonya with MATE desktop on a P71 uusing the dock with two external monitors with nvidia 384.90 and the discrete graphics pre-selected in the BIOS.

When I started this, the first hing I noticed was that having a monitors.xml would cause the screen to flicker, so I had to run everything without it. Then, after boot, I had to put it back to suspend once before I started working, since after the first suspend it would change the SEQUENCE of the monitors. Afterwards, the sequence would remain. So that worked for a while.

After the upgrade to the latest Nvidia driver on October 26th, the sequence was not changed anymore, so that was actually good. This worked for four days, I think. Now, I get the effect mentioned in a post a bit further up: wheneever I try to wake the laptop up from suspend (or hibernate), I just get a flickering and I have to reboot - obviously not what one would want.

I also think that XUbuntu is a bit minimalistic, but if it saves me this headache, I might give that a try. This is really annoying!

[chessplayer (chessplayer)]

So, I finally had the time to test Xubuntu with the dock and indeed, it works the way I want it to. Almost, that is, since when I resume work after a suspend, 4 identical windows open offering display settings. So I have to close these, but I can live with that.

Seeing that my absolute must have, namely a searchable menu, is now provided by Xubuntu by means of the whisker menu, I guess I will stick to it.

[fabio.hipolito (fabio-hipolito)]

Same problems with nvidia-384.90 on both ubuntu-gnome 17.04 and 17.10. The workarounds for 17.04 still minimize part of the issues on 17.10.

[fabio.hipolito (fabio-hipolito)]

At least for ubuntu-gnome 17.04, the desktop can be recovered by forcing gnome to restart, and thus avoid a machine reboot or log out/in

The following appears to work, at least most of the times:
1. open the command prompt with Alt+F2 and type r
2. connect laptop with the docking station and press enter to force the gnome restart

[shaneonabike (shanebill)]

This has been my solution since 16.04 (as noted above). But really it should detect that it's coming back out of sleep.

[fabio.hipolito (fabio-hipolito)]

The status has improved significantly in ubuntu 17.10 with nvidia-384.111-0ubuntu0.17.10.1, but it is not perfect yet.

At this moment I logout/login or lock while plugged to the dock and using multiple screens, works seemingly. Break downs occur at less than 5%

Un-docking and docking success rate is lower, but works most of the time.

Thanks to all involved for the improvements.

[amias (amias)]

am seeing this on ubuntu 18.04 with a dell m3800 and OWC thunderbolt 2 dock and 4k iiyama 40 inch screen attach via displayport.

My internal 13inch 4k screen is initialised immediately but the external monitor tries to sync but often times out.

It looks to me as if this is just an issue with timeouts , surely the driver can made aware that its trying to sync and external screen so should allow longer. This seems to be excerbated by my having different scaling levels on each screen

Sometimes my laptop screen comes back with a strip missing down the side or the the external monitor is at the wrong resolution.

EVERY time it randommly rearranges my windows and triggers a bug with scaling in some qt5 apps (virtualbox , notes) where they get scaled up .

Its very annoying having to keep re-arangeing things.

I tried setting monitors.xml to be readonly and it complains , why can i not fix my layouts ?

[TomasHnyk (sup)]

Hm, I am on 18.04 on Thinkpad 530 and Mini Dock Plus Series 3 with three monitors and I have a very mild version of this problem. Only after suspend, the two monitors that are connected via the dock flicker (either show screen corruption or repeatedly blank in and blank out). The one connected to laptop's miniDP port works fine.

However, I only need to issue xset dpms force standby to make them turn off and then move a mouse to make them turn on again. Another alternative is pressing alt+ctrl+F6 to go to tty6 and then alt+ctrl+F7 to go back to graphical interface. Still inconvenient though.

[TomasHnyk (sup)]

Version of my drivers is 430, it did the same on 390, using the prorietary Nvidia drivers

[Erez (erez-hadad)]

Here is my current method of getting multi-monitor setup working, regardless of the monitor type or connection (Thinkpad P50, Ubuntu 16.04.6, driver 384.30):

Initial setup:
--------------
1. Connect your external monitors to your docking station and to electric power, while your Ubuntu desktop is in *locked* screen (or "switch user"). This is important to avoid screen corruption!
1. Edit ~/.config/monitors.xml *manually* to get the exact configuration you want.
2. Save a copy of monitors.xml in case it gets later changed by auto-detection.
3. Connect each of your external monitors to a *switched* power socket - so that you can turn off your monitor completely without disconnecting it from the docking station. NOTE: If the monitor is powered but soft-off (using the monitor's power button), it will still communicate with card, so using the switched socket, you make sure it is completely off.
4. Add to your desktop startup apps the command "/usr/bin/xset -dpms". This makes sure that when screens are darkened after locking, they don't use DPMS, which is faulty and breaks the multi-monitor setup. Instead, software is used to darken the screen, with mouse cursor still visible.

Docking:
--------
1. Switch off the power supply of all the external monitors, using the power socket switch.
2. Dock your laptop, resume operation, still in *locked screeen* - it now runs stand-alone.
3. Flip on the power switch of the external monitors.
4. If all goes ok, your setup will resume correctly - you will see displays being extended or replicated as set in your monitors.xml . Then you can log in and continue work.
5. Sometimes, setup will not resume correctly on first flip. No worries. Just turn off the switch again, wait a few seconds and try again. In my experience, I don't need to do this more than 3 times, and those are rare occasions. If it doesn't work on the 3rd attempt

Un-docking:
-----------
1. Suspend your laptop
2. Undock.

As a general rule: when you connect your laptop to a new external display (e.g., in a meeting room, preparing to project), ALWAYS lock your desktop first before connecting. From my experience, this guarantees much better display stability and smooth user experience. Do expect that when connecting a new display, the net setup will be somewhat arbitrary - whether extending your current display or replicating it. However, this is still easily manageable e.g., when projecting.