SetMetricScale: an audit that finds the widgets ignoring it, and the first fixes

[tannevaled]

SetMetricScale is documented as the thing a HiDPI host sets once so that "the whole toolkit lays out and paints crisp at that resolution". Ten files out of a hundred and sixty route anything through it. The rest carry raw pixel constants that stay put on a screen with twice the pixels, so the chrome shrinks to half its intended size while the window around it is sharp — and nothing anywhere says so.

Now that all four go-widgets/window back-ends can hand a widget tree a 2× framebuffer, this is the half that was missing.

The instrument

TestMetricScaleAudit finds them, instead of my reading a hundred files and guessing which numbers are pixels. It draws a widget at scale 1 and at scale 2 and compares the run lengths along three rows and three columns: a widget whose metrics scale gives the same runs, twice as wide.

Three things it had to learn, each from being wrong:

• Widths, not boundary positions. Positions move by one pixel when a one-pixel border fails to scale, which is indistinguishable from the rounding of an odd metric — that version declared the very defect this was written to find to be within tolerance.
• Runs no wider than one logical pixel are dropped. An anti-aliased edge is a coverage ramp, not a metric, and at twice the scale it may resolve into two steps where it had one. In logical units, because a border that does scale is one pixel at 1× and two at 2×.
• Three lines per axis, reported only when all three disagree. A metric that does not scale is wrong on every line; a diagonal crossing an exact scanline is not, and calling that a defect would have me contorting a widget to satisfy a probe.

It measures chrome, not text: type carries its own scale and a centred baseline rounds. Whether text should follow the metric scale is a real question and a separate one.

The fixes, over eighteen widgets

• strokeRect / strokeRoundRect ask for a stroke one logical pixel wide. This one line fixes Button, Card, ProgressBar, Chip, Entry, Alert, Banner, Frame and every other framed widget at once. It needed painter#17 (v0.11.0), which until now took a stroke width and discarded it.
• CheckButton: the box, the label gap, and the checkmark — a hard-coded twelve-pixel path that sat in the corner of a box twice its size. The classic 12px tick is kept byte-for-byte for the classic 12px box.
• Switch: the knob inset. Expander/Accordion: the header height, through a new ExpanderHeaderHeight(). Gauge: the arc thickness.

Control

	audit
with the fixes	green, 18 widgets
revert the stroke width	5 of 6 original widgets flagged
revert only the checkbox scaling	exactly CheckButton flagged

The catalogue is eighteen widgets of roughly a hundred and forty. Growing it is the mechanism for the rest: every widget added either passes or names its own defect.

🤖 Generated with Claude Code