X is misrendering a rectangle around my mouse pointer

A square area around my mouse pointer is being rendered incorrectly. Instead of rendering the window it is supposed to render, it seems to be rendering the window underneath instead. It is very clear in this screenshot, which omits the mouse pointer itself, but in which you can see part of my gray desktop background “through” the all-white Konsole window: If I log out and then log in again, without rebooting, the problem goes away. Here are a couple more screenshots, cropped to the areas of interest: Here the pointer is hovering over the Firefox tab with the blue “J”; the black popup is the tool tip for the tab, part of it is missing and you can see the main Firefox window instead: Here the pointer is hovering over the Firefox “try again” button, but X has rendered a bit from the Spectacle window that is underneath Firefox: My X server is:

X.Org X Server 1.19.3
Release Date: 2017-03-15
X Protocol Version 11, Revision 0
Build Operating System: Linux 4.4.0-87-generic x86_64 Ubuntu
Current Operating System: Linux avocet 4.10.0-33-generic #37~16.04.1-Ubuntu SMP Fri Aug 11 14:07:24 UTC 2017 x86_64
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-4.10.0-33-generic root=UUID=19f82f72-a807-4624-a7aa-8297febd0f5a ro quiet splash vt.handoff=7
Build Date: 25 July 2017  01:30:08PM
xorg-server 2:1.19.3-1ubuntu1~16.04.2 (For technical support please see http://www.ubuntu.com/support)  
Current version of pixman: 0.33.6

The OS is:

Distributor ID: Ubuntu
Description:    Ubuntu 16.04.3 LTS
Release:        16.04
Codename:       xenial

The window system is KDE. The hardware is a Lenovo Thinkpad T460p laptop. I'm not sure which parts of the system configuration are relevant. Here's the compositor configuration: Here's part of the output of lshw that seems relevant: *-pci:1 description: PCI bridge product: Sky Lake PCIe Controller (x4) vendor: Intel Corporation physical id: 1.2 bus info: pci@0000:00:01.2 version: 07 width: 32 bits clock: 33MHz capabilities: pci pm msi pciexpress normal_decode bus_master cap_list configuration: driver=pcieport resources: irq:121 ioport:d000(size=4096) memory:f1000000-f1ffffff ioport:c0000000(size=301989888) *-display description: 3D controller product: GM108M [GeForce 940MX] vendor: NVIDIA Corporation physical id: 0 bus info: pci@0000:02:00.0 version: a2 width: 64 bits clock: 33MHz capabilities: pm msi pciexpress bus_master cap_list configuration: driver=nvidia latency=0 resources: irq:130 memory:f1000000-f1ffffff memory:c0000000-cfffffff memory:d0000000-d1ffffff ioport:d000(size=128) *-display description: VGA compatible controller product: Intel Corporation vendor: Intel Corporation physical id: 2 bus info: pci@0000:00:02.0 version: 06 width: 64 bits clock: 33MHz capabilities: pciexpress msi pm vga_controller bus_master cap_list rom configuration: driver=i915 latency=0 resources: irq:126 memory:f0000000-f0ffffff memory:e0000000-efffffff ioport:e000(size=64) memory:c0000-dffff How should I proceed in trying to diagnose and fix this? If I should be reporting a bug, then to whom should I report it, and how? [ Added 2017-SEP-18: This appears to be a bug, [described here](https://www.kubuntuforums.net/showthread.php/72149-Mouse-artifact-introduced-in-7-26-update) . Restarting sddm with sudo systemctl restart sddm works around the problem. ]