fix(plot): text and leader background masks plot paper-white, not black

The mask is filled with the canvas colour; plotting carried that dark
colour (or the light-to-black rule) onto the sheet.

Fixes #1072

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/update/file.rs

@@ -412,12 +412,24 @@ fn plot_scene_content(
                     }))
     });
     model_wires.retain(|wire| wire.plot_visible);
+    // A text / leader background mask is filled with the canvas colour so it
+    // hides what is behind it; on paper that colour is the sheet's, not the
+    // dark screen's. Mark such fills so the export paints them paper-white
+    // instead of plotting the canvas colour (or turning it black). (#1072)
+    let canvas = scene.current_bg();
     let with_depth = |wires: Vec<crate::scene::WireModel>| {
         let depths = scene.plot_wire_depths(&wires);
         wires
             .into_iter()
             .zip(depths)
             .map(|(mut wire, draw_depth)| {
+                if !wire.fill_tris.is_empty() && wire.color[..3] == canvas[..3] {
+                    wire.bg_adapt = Some(Box::new(crate::scene::model::wire_model::BgAdaptInputs {
+                        raw_color: wire.color,
+                        canvas_color: true,
+                        ..Default::default()
+                    }));
+                }
                 plot_owner_aci(scene, &mut wire);
                 crate::io::pdf_export::PlotWire { wire, draw_depth }
             })

src/io/pdf_export.rs

@@ -1133,21 +1133,26 @@ fn emit_wire_fills(
         if a < 0.01 {
             continue;
         }
-        let mut screening = 1.0;
-        let mut color_overridden = false;
-        if let Some(table) = plot_style {
-            if wire.aci > 0 {
-                if let Some(color) = table.resolve_color(wire.aci) {
-                    [r, g, b] = color;
-                    color_overridden = true;
+        if wire.bg_adapt.as_deref().is_some_and(|adapt| adapt.canvas_color) {
+            // A background mask: it covers with the paper itself.
+            [r, g, b] = [1.0, 1.0, 1.0];
+        } else {
+            let mut screening = 1.0;
+            let mut color_overridden = false;
+            if let Some(table) = plot_style {
+                if wire.aci > 0 {
+                    if let Some(color) = table.resolve_color(wire.aci) {
+                        [r, g, b] = color;
+                        color_overridden = true;
+                    }
+                    screening = table.resolve_screening(wire.aci);
                 }
-                screening = table.resolve_screening(wire.aci);
             }
+            if !color_overridden {
+                [r, g, b] = adapt_text_color([r, g, b]);
+            }
+            [r, g, b] = plotted_color([r, g, b], a, screening, options);
         }
-        if !color_overridden {
-            [r, g, b] = adapt_text_color([r, g, b]);
-        }
-        [r, g, b] = plotted_color([r, g, b], a, screening, options);
         ops.push(Op::SetFillColor {
             col: Color::Rgb(Rgb {
                 r,

src/scene/mod.rs

@@ -8573,7 +8573,7 @@ impl Scene {
     }
 
     /// Background colour for the current layout (model vs paper).
-    fn current_bg(&self) -> [f32; 4] {
+    pub(crate) fn current_bg(&self) -> [f32; 4] {
         if self.current_layout != "Model" {
             self.paper_bg_color
         } else {