Plot: thin lineweights keep their true width (#1749)

Every weight under 0.26 mm was clamped to the screen's 1 px floor before
the PDF and SVG exporters saw it, so 0.00, 0.09, 0.13 and 0.25 mm all
plotted alike. The floor now applies on the GPU only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/control/mod.rs

@@ -2907,8 +2907,8 @@ mod tests {
         assert_eq!(response["status"], "completed", "{response}");
 
         // The plotted PDF carries two distinct pen widths — the per-layer
-        // 0.13 mm / 0.50 mm hierarchy survives the plot style (thin clamps to
+        // 0.13 mm / 0.50 mm hierarchy survives the plot style at its true
-        // the 1 px floor, so the ratio is the clamped-pixel ratio 1.889).
+        // ratio; the screen's 1 px floor does not reach the plot (#1749).
         let text =
             crate::io::pdf_export::pdf_stream_text(&std::fs::read(&pdf).expect("pdf written"));
         let tokens: Vec<&str> = text.split_whitespace().collect();
@@ -2931,7 +2931,7 @@ mod tests {
             "two layer weights in the plotted PDF: {widths:?}"
         );
         assert!(
-            (widths[1] / widths[0] - 1.889).abs() < 0.15,
+            (widths[1] / widths[0] - 50.0 / 13.0).abs() < 0.15,
             "0.50 mm vs 0.13 mm hierarchy: {widths:?}"
         );
         let _ = std::fs::remove_file(&pdf);

src/scene/pipeline/circle_gpu.rs

@@ -264,7 +264,7 @@ pub fn extract_circle_instance_from_geom_indexed(
     }
 
     let (ch, cl) = split_ds_xyz(center[0], center[1], center[2]);
-    let hw = wire.line_weight_px * 0.5;
+    let hw = wire.line_weight_px.max(1.0) * 0.5;
     let pat0 = [
         wire.pattern[0],
         wire.pattern[1],

src/scene/pipeline/ellipse_gpu.rs

@@ -235,7 +235,7 @@ pub fn extract_ellipse_instance_from_geom(
     }
 
     let (ch, cl) = split_ds_xyz(center[0], center[1], center[2]);
-    let hw = wire.line_weight_px * 0.5;
+    let hw = wire.line_weight_px.max(1.0) * 0.5;
     let pat0 = [
         wire.pattern[0],
         wire.pattern[1],

src/scene/pipeline/wire_gpu.rs

@@ -863,7 +863,7 @@ pub(crate) fn emit_wire_packed(
         wire.pattern[6],
         wire.pattern[7],
     ];
-    let half_width = wire.line_weight_px * 0.5;
+    let half_width = wire.line_weight_px.max(1.0) * 0.5;
     let n = wire.points.len();
     let seg_count = n.saturating_sub(1);
     if seg_count == 0 {
@@ -965,7 +965,7 @@ pub(crate) fn emit_wire_native(
             wire.pattern[6],
             wire.pattern[7],
         ],
-        half_width: wire.line_weight_px * 0.5,
+        half_width: wire.line_weight_px.max(1.0) * 0.5,
         pattern_length: wire.pattern_length,
         draw_depth,
         align_end,

src/scene/view/render.rs

@@ -3589,9 +3589,11 @@ pub struct InheritStyle {
 }
 
 /// Convert a concrete (already layer-resolved) lineweight to display pixels.
+/// Kept below 1 px for thin weights so plots and SVG get the true width;
+/// the GPU paths draw at least 1 px (#1749).
 pub(crate) fn lineweight_to_px(lw: &LineWeight) -> f32 {
     lw.millimeters()
-        .map(|mm| (mm as f32 * MM_TO_PX).max(1.0))
+        .map(|mm| mm as f32 * MM_TO_PX)
         .unwrap_or(1.0)
 }