The cursor snagging at the dual-monitor boundary — an NVIDIA + Wayland bug

Environment: Kubuntu 26.04 LTS · KDE Plasma 6 · NVIDIA nvidia-driver-595-open · 2560×1440 dual monitors

TL;DR

If the cursor snags crossing between two monitors on NVIDIA proprietary + Wayland, before digging into KWin settings, test whether it reproduces in an X11 session. If it's gone under X11, that confirms an NVIDIA+Wayland issue.

# If there's no "Plasma (X11)" entry on the login screen, you need both packages
sudo apt install kwin-x11 plasma-session-x11

The symptom

With two 2560×1440 monitors side by side, the user reported that moving the cursor slowly across the boundary felt like it snagged every time. It was worst when several windows were pinned to the screen edges — as if the cursor got grabbed by a resize handle and needed extra mouse movement to break free.

The misdiagnosis: "it must be catching on KWin tiling zones"

In kwinrc, each screen had KWin Quick Tiling zone settings (the translucent areas that appear when you drag a window to an edge) left as a 25%/50%/25% layout. I figured the resize handles at those zone boundaries were the snag. That was wrong.

I asked the user to actually drag a window to a screen edge, and no translucent zone overlay appeared at all — the tile editor feature was simply disabled. The settings lingering in kwinrc were just dead data. I'd been barking up the wrong tree entirely.

Root cause: it only reproduces under Wayland

The setup was nvidia-driver-595-open + a KDE Plasma 6 Wayland session. What the A/B test below establishes is that the symptom only occurs under Wayland — no more than that. Which layer actually misbehaves as the cursor crosses between outputs (driver or compositor, cursor-plane handling or something else) is something I never pinned down. I could offer a plausible story, but it would go past what was observed here, so I'll leave it out.

Confirming and fixing: switch to an X11 session

I asked the user to log out and log back in via the gear icon as a "Plasma (X11)" session, then reproduce the same motion. Under X11 the symptom was completely gone. Same hardware, same layout, only the session changed — so the cause clearly lives somewhere on the Wayland path. And for now, running an X11 session is the surest fix.

But logging into X11 can be a problem: a default Ubuntu/Kubuntu install may not ship the X11 session at all (it didn't here — /usr/share/xsessions/ was missing entirely). Two packages are needed.

sudo apt install kwin-x11 plasma-session-x11

Installing only kwin-x11 gives you the window-manager binary; what actually makes a "Plasma (X11)" entry appear on the login screen is plasma-session-x11 (the Wayland side has its own plasma-session-wayland doing that role). It's easy to conflate the two.

Takeaways / checklist

For cursor/window oddities on multi-monitor, it's worth testing the Wayland vs X11 session difference before digging into KWin settings — especially with a proprietary NVIDIA driver.
KWin Quick Tiling zone settings (kwinrc) can linger as dead data even when the feature is off — don't assume a feature is "active now" just because a value exists; reproduce it to confirm.
If there's no "Plasma (X11)" login option, install plasma-session-x11 as well as kwin-x11.

This X11 switch was also what confirmed the Han/Yeong key was actually sending Alt_R (that bug reproduced under X11 too), and the dummy HDMI plug showing up as a phantom third monitor came out of the same multi-monitor cleanup.