The ToolBtn row used an absolute `x: 8/10px` offset together with
`width: preferred + padding`, which pushed the icon (and label) off-centre —
the Filter and Select buttons read as misaligned. Replace the x offset with
symmetric padding-left/right inside the HorizontalLayout and size the button to
the layout's natural width, so the glyph is centred deterministically.
cc
vicrodh
47fdfe6622