Radial dimensions: text on the line, a short leader when it moves out

The text of a radius or diameter constraint sits on its dimension line,
which breaks around it, and no centre mark is drawn while that line is
there. Text that no longer fits inside moves beside the circle at the end
of a short leader that leaves the rim, elbows and hooks under the text,
with the arrowheads keeping the circle. A dynamic dimension now keeps
only its text at a constant screen size; arrowheads and extension lines
keep the style's size in drawing units.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/scene/parametric_constraints.rs

@@ -2388,6 +2388,7 @@ impl super::Scene {
                     ov::DIMEXO,
                     ov::DIMEXE,
                     ov::DIMASZ,
+                    ov::DIMTAD,
                     ov::DIMTIH,
                     ov::DIMTOH,
                 ] {
@@ -2426,7 +2427,7 @@ impl super::Scene {
                 continue;
             }
             let style_name = dimension.base().style_name.clone();
-            let text_height = self
+            let style = self
                 .document
                 .dim_styles
                 .iter()
@@ -2434,23 +2435,40 @@ impl super::Scene {
                     style.name.eq_ignore_ascii_case(&style_name)
                         || (style_name.trim().is_empty()
                             && style.name.eq_ignore_ascii_case("Standard"))
-                })
+                });
+            let text_height = style
                 .map(|style| style.dimtxt)
                 .filter(|height| *height > 1e-9)
                 .unwrap_or(0.18);
+            let size_of = |value: Option<f64>, fallback: f64| {
+                value.filter(|size| *size > 1e-9).unwrap_or(fallback)
+            };
+            let arrow_size = size_of(style.map(|style| style.dimasz), text_height);
+            let extension_over = size_of(style.map(|style| style.dimexe), text_height * 0.5);
+            let extension_offset = size_of(style.map(|style| style.dimexo), text_height * 0.25);
             let scale = f64::from(wpp) * f64::from(DYNAMIC_DIMENSION_TEXT_PX) / text_height;
             use crate::entities::dim_override as ov;
             let xdata = &dimension.base().common.extended_data;
             let current = ov::real(xdata, ov::DIMSCALE);
-            // Text gap, extension offset and overshoot and arrow size follow
-            // the text height on screen (the reference's ratios), whatever
-            // the style says in drawing units.
+            // The gap hugs the text, so it follows the text's screen size.
+            // Arrowheads and extension lines keep the style's size in
+            // drawing units: dividing by the screen factor the style's
+            // DIMSCALE re-applies leaves them unchanged as the view zooms.
             let sizes = [
                 (ov::DIMGAP, text_height * 0.25),
-                (ov::DIMEXO, text_height * 0.25),
-                (ov::DIMEXE, text_height * 0.5),
-                (ov::DIMASZ, text_height),
+                (ov::DIMEXO, extension_offset / scale),
+                (ov::DIMEXE, extension_over / scale),
+                (ov::DIMASZ, arrow_size / scale),
             ];
+            // A radius or diameter constraint reads on its own dimension
+            // line, which breaks around the text, so the text is centred on
+            // it rather than lifted above it.
+            let radial = matches!(
+                dimension,
+                acadrust::entities::Dimension::Radius(_)
+                    | acadrust::entities::Dimension::Diameter(_)
+            );
+            let centred = !radial || ov::int(xdata, ov::DIMTAD) == Some(0);
             let sizes_set = sizes.iter().all(|(code, size)| {
                 ov::real(xdata, *code).is_some_and(|value| (value - size).abs() < 1e-12)
             });
@@ -2463,9 +2481,21 @@ impl super::Scene {
             );
             let horizontal = angular
                 || (ov::int(xdata, ov::DIMTIH) == Some(1) && ov::int(xdata, ov::DIMTOH) == Some(1));
-            if current.is_some_and(|value| (value - scale).abs() < 1e-9) && horizontal && sizes_set {
+            if current.is_some_and(|value| (value - scale).abs() < 1e-9)
+                && horizontal
+                && sizes_set
+                && centred
+            {
                 continue;
             }
+            if radial && !centred {
+                ov::set(
+                    &mut self.document,
+                    handle,
+                    ov::DIMTAD,
+                    Some(acadrust::xdata::XDataValue::Integer16(0)),
+                );
+            }
             ov::set(
                 &mut self.document,
                 handle,