"disable touchpad when mouse is plugged in" also disables touchpad when keyboard is plugged in.

[Ian Turner]

STEPS TO REPRODUCE:
1. Open System Settings -> Input Devices -> Touchpad -> Enable/Disable Touchpad, ensure "Disable touchpad when mouse is plugged in" is checked.
2. Connect external USB keyboard (I tested with MS Natural Keyboard, USB ID 045e:00db)
3. Touchpad is disabled.
4. Disconnect external keyboard
5. Touchpad is re-enabled.

EXPECTED BEHAVIOR: Touchpad would remain enabled as long as an external pointing device is not connected.

WORKAROUND: Uncheck "Disable touchpad when mouse is plugged in".

HARDWARE NOTES: Tested on an HP ProBook 640 G1 and a Microsoft Natural Keyboard 4000 v1.0. This keyboard does not have a built-in trackpad or other pointing device; and according to "lsusb -v -d 045e:00db" it supports both bInterfaceProtocol 1 "Keyboard" and bInterfaceProtocal 0 "None".

LOG MESSAGES: I see this in syslog, which could be related. The keyboard is registered as both an HID "Keyboard" and as an HID "Device'.
Jul 17 10:31:18 cue kernel: [838924.674754] input: Microsoft Natural® Ergonomic Keyboard 4000 as /dev
ices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.0/0003:045E:00DB.000C/input/input290
Jul 17 10:31:18 cue kernel: [838924.733606] microsoft 0003:045E:00DB.000C: input,hidraw0: USB HID v1.
11 Keyboard [Microsoft Natural® Ergonomic Keyboard 4000] on usb-0000:00:14.0-2/input0
Jul 17 10:31:18 cue kernel: [838924.743708] input: Microsoft Natural® Ergonomic Keyboard 4000 as /dev
ices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.1/0003:045E:00DB.000D/input/input291
Jul 17 10:31:18 cue kernel: [838924.801195] microsoft 0003:045E:00DB.000D: input,hidraw1: USB HID v1.
11 Device [Microsoft Natural® Ergonomic Keyboard 4000] on usb-0000:00:14.0-2/input1

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: plasma-desktop 4:5.12.6-0ubuntu0.1
ProcVersionSignature: Ubuntu 4.15.0-24.26-generic 4.15.18
Uname: Linux 4.15.0-24-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
Date: Tue Jul 17 10:35:18 2018
InstallationDate: Installed on 2018-03-23 (116 days ago)
InstallationMedia: Kubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180306.1)
ProcEnviron:
 LANGUAGE=
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: plasma-desktop
UpgradeStatus: No upgrade log present (probably fresh install)

[Scarlett Gately Moore (scarlettmoore)]

This bug is very old, if it is still an issue please update the bug with information from a supported release.