(In reply to akb825 from comment #0)
> SUMMARY
> After updating to the latest version of Plasma/KDE, the touchpad on my
> laptop gets disabled automatically due to KDE detecting that a mouse is
> plugged in, despite the fact that there is no mouse present. Upon further
> investigation, my touchpad is being detected as both a touchpad and mouse
> with separate device entries. It appears that the KDE control panel for
> touchpad input was recently changed and no longer gives an option for
> whether or not to disable the touchpad when a mouse is connected: as a
> result my touchpad is *always* disabled with no option of re-enabling it.
> 
> STEPS TO REPRODUCE
> 1. Boot to login screen with no mouse plugged in. Touchpad works.
> 2. Log in. Touch pad is disabled.
> 
> OBSERVED RESULT
> A notification pops up saying that the trackpad was disabled because a mouse
> was plugged in.
> 
> EXPECTED RESULT
> Touchpad continues to work unless a physical mouse is plugged in.
> 
> SOFTWARE/OS VERSIONS
> Linux/KDE Plasma: Arch Linux, fully up to date
> KDE Plasma Version: 5.17.4
> KDE Frameworks Version: 5.64.0
> Qt Version: 5.13.2
> 
> ADDITIONAL INFORMATION
> The laptop I'm using is a Dell XPS 15 9570.
> 
> The relative output from /proc/bus/input/devices is:
> I: Bus=0018 Vendor=06cb Product=7a13 Version=0100
> N: Name="SYNA2393:00 06CB:7A13 Mouse"
> P: Phys=i2c-SYNA2393:00
> S:
> Sysfs=/devices/pci0000:00/0000:00:15.1/i2c_designware.1/i2c-6/i2c-SYNA2393:
> 00/0018:06CB:7A13.0002/input/input23
> U: Uniq=
> H: Handlers=event17 mouse4 
> B: PROP=0
> B: EV=17
> B: KEY=30000 0 0 0 0
> B: REL=3
> B: MSC=10
> 
> I: Bus=0018 Vendor=06cb Product=7a13 Version=0100
> N: Name="SYNA2393:00 06CB:7A13 Touchpad"
> P: Phys=i2c-SYNA2393:00
> S:
> Sysfs=/devices/pci0000:00/0000:00:15.1/i2c_designware.1/i2c-6/i2c-SYNA2393:
> 00/0018:06CB:7A13.0002/input/input24
> U: Uniq=
> H: Handlers=event18 mouse5 
> B: PROP=5
> B: EV=1b
> B: KEY=e520 10000 0 0 0 0
> B: ABS=2e0800000000003
> B: MSC=20
> 
> The relative output from running the command libinput list-devices is:
> Device:           SYNA2393:00 06CB:7A13 Mouse
> Kernel:           /dev/input/event17
> Group:            9
> Seat:             seat0, default
> Capabilities:     pointer 
> Tap-to-click:     n/a
> Tap-and-drag:     n/a
> Tap drag lock:    n/a
> Left-handed:      disabled
> Nat.scrolling:    disabled
> Middle emulation: n/a
> Calibration:      n/a
> Scroll methods:   button
> Click methods:    none
> Disable-w-typing: n/a
> Accel profiles:   flat *adaptive
> Rotation:         n/a
> 
> Device:           SYNA2393:00 06CB:7A13 Touchpad
> Kernel:           /dev/input/event18
> Group:            9
> Seat:             seat0, default
> Size:             102x77mm
> Capabilities:     pointer gesture
> Tap-to-click:     disabled
> Tap-and-drag:     enabled
> Tap drag lock:    disabled
> Left-handed:      disabled
> Nat.scrolling:    disabled
> Middle emulation: disabled
> Calibration:      n/a
> Scroll methods:   *two-finger edge 
> Click methods:    *button-areas clickfinger 
> Disable-w-typing: enabled
> Accel profiles:   none
> Rotation:         n/a

Can you see if installing the xf86-input-synaptics brings back the option to disable touchpad while mouse is connected.

> Can you see if installing the xf86-input-synaptics brings back the option to
> disable touchpad while mouse is connected.

Thank you, that did restore the ability to adjust when the touchpad is enabled. I was able to add the "mouse" entry for the touchpad to the ignore list without fully disabling the feature. I need to use an XOrg config to customize the settings, but this was the same as the previous behavior, so it's at least a usable workaround for now.

Same issue, on an XPS 15 9570 myself. Here's the workaround I'm using:

$ cat /etc/X11/xorg.conf.d/99-custom.conf 
Section "InputClass"
        Identifier "Ignore synaptics mouse"
        MatchProduct "SYNA2393:00 06CB:7A13 Mouse"
        MatchIsPointer "on"
        Option "Ignore" "on"
EndSection

After updating today having xf86-input-synaptics installed no longer works around the bug. Applying the X11 config by Mads was able to restore the touchpad.

To provide an update for this issue, the X11 workaround Mads posted is inconsistent. Sometimes it still disables the touchpad on boot and requires one or two reboots before it works. I've kept up to date based on Arch's packages, so I'm currently on Plasma 5.19.3.

From what I can tell the workaround listed by Mads is no longer doing anything. Having the workaround or not has the same behavior: sometimes when I boot (maybe 25-50% of the time) it will disable the trackpad, other times it won't. Logging off and logging back on doesn't seem to change anything, a full reboot is required. (though there's probably some magic command that could fix it as well)

Would it be possible to at least bring back the option to not disable the trackpad when it detects a mouse?

After some investigation, it looks like the current issue is caused because there's a second touchpad device (PS/2 Synaptics TouchPad) that shows up in the xinput device list. The ordering of the list is inconsistent between boots, and it will incorrectly disable the touchpad when the PS/2 Synaptics TouchPad is on top.

When I disable this extra touchpad device, it looks like it works around the issue. Therefore, the new workaround for the Xorg config is:

Section "InputClass"
    Identifier "Ignore synaptics touchpad"
    MatchProduct "PS/2 Synaptics TouchPad"
    MatchIsTouchpad "on"
    Option "Ignore" "on"
EndSection

Just to note, I do not have the xf86-input-synaptics package installed, so the Synaptics device seems to be detected even with the default X11 packages.

This seems to be a constantly moving target. I additionally need to disable PS/2 Generic Mouse to have the touchpad reliably work:

Section "InputClass"
	Identifier "Ignore synaptics mouse"
	MatchProduct "PS/2 Generic Mouse"
	MatchIsPointer "on"
	Option "Ignore" "on"
EndSection

Nevermind, it isn't consistent even with that change: XOrg will still sometimes keep the device enabled, even when verifying that the device name listed in xinput matches one of the ones I've disabled through an XOrg config.

Related problem here, though opposite, and less inconvenient, though still annoying, when I upgraded my laptop:

The "Disable touchpad when mouse is plugged in" was off (and disappeared), and no way to turn it on.

I too noticed this option(*) went away in recent updates to OpenSUSE Tumbleweed's KDE packaging. I couldn't find the option to disable touchpad upon mouse plugged in.

When I installed xserver-xorg-input-synaptics package, the option is restored(*).

Though it does not solve the more complex problem described here, nvtl IMHO, the xserver-xorg-input-synaptics package *should* be installed by default (all distros): This would at least reduce one level of complexity, putting everyone on the same page.

(*) Start --> Settings --> System Settings --> Hardware/Input Devices --> Touchpad --> Enable/Disable Touchpad --> [x] Disable touchpad when mouse is plugged in.

N.B. - On my OpenSUSE Tumbleweed distro, the package is slightly renamed to "xf86-input-synaptics". I should have mentioned that, above.

For the original reporter of the issue: this bug report concerns the Synaptics version of the Touchpad page, which is no longer supported in Plasma 6. Can you please see if the issue still happens when using Libinput? Thanks!

For other people: please open new bug reports to report new issues. Thanks!

After updating to Plasma 6 and switching to a Wayland session and libinput, everything seems to be working as expected. IIRC I earlier opted to use Synaptics because the libinput driver didn't support drag-lock, but this is now supported under Plasma 6.

Great!