The cursor jumps between a 4K and a 1440p monitor. Here is why.

Windows measures screens in pixels, not centimetres

A 27-inch 4K panel is 3840 × 2160 pixels; a 27-inch 1440p panel is 2560 × 1440. Both are 597 mm wide. Windows knows nothing about the 597 mm: in Display settings the 4K rectangle is drawn one and a half times as large as the 1440p one, and the cursor crosses where the rectangles touch. Set the 4K screen to 150 % scaling and the desktop looks the same size, but the crossing is still computed on the raw pixels.

The result: a cursor that leaves the 1440p screen at its bottom edge arrives on the 4K screen only two thirds of the way down, and the last third of the 4K edge is a dead zone where the cursor stops.

Fix on one PC: align the rectangles by hand

Open Settings → System → Display, drag the smaller rectangle so that its top or centre matches the way the monitors really stand, and use the same scaling factor on both if you can live with it. It reduces the offset but cannot remove the dead zone, because the two edges have different pixel lengths.

Across two PCs, pixel-based KVM tools inherit the problem

Mouse Without Borders, ShareMouse, Synergy and Barrier hand the cursor over at the pixel edge of the screen it leaves and place it at the same pixel fraction on the screen it enters. Two 4K screens are 7680 px wide, two 1440p screens 5120 px: the top row covers only two thirds of the bottom row, and everything to the right of that is a wall. On top of that, the cursor moves at a different speed on each PC, because a mouse count means one pixel on both.

What DeskStitch does differently

Every monitor reports its physical size in its EDID. DeskStitch reads it, lays the screens out on a desk map in millimetres, computes crossings in millimetres and converts mouse travel to millimetres and back. A 4K and a 1440p panel of the same size overlap fully, the cursor arrives at the physical height it left, and a hand movement covers the same distance on both PCs. If a monitor reports a wrong size, drag its tile in the layout editor; edges snap, arrow keys nudge by a millimetre.

Checklist

• Same physical size, different resolution: the classic case; a millimetre layout fixes it completely.
• Different physical size: align the edges you actually cross; a millimetre layout still keeps the speed equal.
• Portrait monitor next to landscape: same story, taller edge, the crossing height is what matters.
• Laptop screen next to a big monitor: expect a wide dead zone by pixels; place the laptop where it stands.