What theme are you using? It's not as bad on my side but still visible if you know it:

The theme I use is "Dark modern".

@MisterMakerNL can you reproduce ?

Is this issue still relevant?

Columns perfect.
Rows has different line width, so it's hard to determine what would be perfect.

OS: Arch Linux (KDE/plasma/xcb)
Architecture: x86_64
Version: 1.1.0dev.39416 (Git) Conda AppImage
Build type: Release
Branch: main
Hash: f6268ffd2877dadb4440ade541e12c388a517d6e
Python 3.11.9, Qt 5.15.13, Coin 4.0.3, Vtk 9.2.6, OCC 7.7.2
Locale: English/United States (en_US)
Stylesheet/Theme/QtStyle: FreeCAD Dark.qss/FreeCAD Dark/Fusion
Installed mods: lattice2 1.0.0; CurvedShapes 1.0.13; MeshRemodel 1.10.35; freecad.gears 1.3.0; fasteners 0.5.33; sheetmetal 0.6.0; Silk 0.1.5; Curves 0.6.51

@tarman3 Seems to be an offset of a pixel on the corner cell!
I also think the cell it is the wrong color. I think we should fix this!

Making it look better was pretty easy fixing the offset isn't.

`/* QHeaderView ------------------------------------------------------------

https://doc.qt.io/qt-5/stylesheet-examples.html#customizing-qheaderview

--------------------------------------------------------------------------- */
QTableCornerButton::section {
padding: -2px;
margin: -2px;
background-color: #333333;
border-radius: 3px; /if you remove this line it breaks it/
border: 2px solid #d70606;
}

QHeaderView {
background-color: transparent;
/* text-align: bottom; */
}

QHeaderView:disabled {
/* background-color: #444444; */
color: #adadad;

}

QHeaderView::section {
background-color: qlineargradient(x1:0, y1:0.3, x2:0, y2:1, stop:0 #333333, stop:1 #252525);
color: #ffffff;
padding: 4px;
}

QHeaderView::section::horizontal {
border-bottom: 1px solid #020202;
border-right: 0px solid transparent;
border-top: 1px solid #020202;
border-left: 1px solid #020202;
}

QHeaderView::section::horizontal::first, QHeaderView::section::horizontal::only-one {
/* border-left: 0px solid #020202; */
}

QHeaderView::section::horizontal:disabled {
color: #adadad;
}

QHeaderView::section::vertical {
border-bottom: 0px solid transparent;
border-right: 1px solid #020202;
border-top: 1px solid #020202;
border-left: 1px solid #020202;
padding-left: 4px;
padding-right: 4px;
}

QHeaderView::section::vertical::first, QHeaderView::section::vertical::only-one {
/* border-top: 2px solid #020202; */
}

QHeaderView::section::vertical:disabled {
border-top: 2px solid #020202;
}
`

I couldn't find this topic anymore tnx Max :)