fix(annotative): start new drawings at the unit-aware annotation scale

A new drawing is metric with metre insertion units, but its annotation
multiplier stayed at Scene::new()'s unitless 1 while opened files and
CANNOSCALE convert paper millimetres into drawing units (0.001 at 1:1).
Annotative text and dimensions were drawn 1000x too large until the
first scale change, then shrank to their real size and looked gone,
also after switching back to 1:1.

One helper now derives the multiplier from the header; new drawings,
file open and every INSUNITS change (UNITS, the units popup, DWGUNITS,
SETVAR) go through it.

Fixes #1674

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

src/app/commands/layers.rs

@@ -893,6 +893,7 @@ impl OpenCADStudio {
 
         self.push_undo_snapshot(i, "DWGUNITS");
         self.tabs[i].scene.document.header.insertion_units = to;
+        self.tabs[i].scene.sync_annotation_scale_from_header();
 
         let factor = units::conversion_factor(from, to).filter(|f| (f - 1.0).abs() > 1e-12);
         let (from_label, to_label) = (units::label(from), units::label(to));

src/app/commands/styleprops.rs

@@ -3088,6 +3088,10 @@ impl OpenCADStudio {
                                 self.delete_objects = value;
                             }
                             if changed {
+                                if name == "INSUNITS" {
+                                    self.tabs[i].scene.sync_annotation_scale_from_header();
+                                    self.tabs[i].scene.invalidate_annotation_dependencies();
+                                }
                                 if matches!(
                                     name.as_str(),
                                     "ZOOMWHEEL"

src/app/update/file.rs

@@ -2115,22 +2115,7 @@ impl OpenCADStudio {
         ) {
             crate::scene::text::ttf_glyph::clear_fallback_cache();
         }
-        // Current model-space annotation scale comes from the drawing's
-        // CANNOSCALEVALUE (paper/drawing factor). Convert its inverse into
-        // drawing units as well: metric annotation sizes are paper millimetres
-        // and imperial annotation sizes are paper inches.
-        // Current model-space annotation scale comes from the drawing's
-        // CANNOSCALEVALUE (paper/drawing factor). Convert its paper unit into
-        // the drawing's INSUNITS as well, so e.g. a metre drawing uses
-        // 0.001 model units for 1 mm of paper at 1:1.
-        let cannoscale_value = self.tabs[i].scene.document.header.annotation_scale_value;
-        let unit_factor = self.tabs[i].scene.annotation_scale_unit_factor();
-
-        self.tabs[i].scene.annotation_scale = if cannoscale_value > 1e-9 {
-            ((1.0 / cannoscale_value) / unit_factor) as f32
-        } else {
-            (1.0 / unit_factor) as f32
-        };
+        self.tabs[i].scene.sync_annotation_scale_from_header();
 
         // Open-time breakdown so regressions are visible immediately.
         // `total` is wall time from the Open click to here (post-xref,

src/app/update/mod.rs

@@ -5442,6 +5442,8 @@ impl OpenCADStudio {
                     header.angle_base = degrees.to_radians();
                 }
                 header.insertion_units = state.insertion_units;
+                self.tabs[i].scene.sync_annotation_scale_from_header();
+                self.tabs[i].scene.invalidate_annotation_dependencies();
                 self.tabs[i].dirty = true;
                 self.tabs[i].scene.bump_geometry();
                 self.refresh_properties();
@@ -6020,6 +6022,8 @@ impl OpenCADStudio {
                 // menu having done nothing. (#668)
                 self.push_undo_snapshot(i, "UNITS");
                 self.tabs[i].scene.document.header.insertion_units = code;
+                self.tabs[i].scene.sync_annotation_scale_from_header();
+                self.tabs[i].scene.invalidate_annotation_dependencies();
                 self.tabs[i].dirty = true;
                 let label = crate::modules::draw::units::label(code);
                 self.command_line.push_output(