diff --git a/crates/openpencil-shell-core/src/document/mutators.rs b/crates/openpencil-shell-core/src/document/mutators.rs
index fe7adc49d..6fd62b0c1 100644
--- a/crates/openpencil-shell-core/src/document/mutators.rs
+++ b/crates/openpencil-shell-core/src/document/mutators.rs
@@ -387,6 +387,18 @@ impl Document {
         }
     }
 
+    /// True when ANY widget occupies the right rail today —
+    /// PropertyPanel (gated on selection) or VariablesPanel
+    /// (gated on a non-empty var table). `canvas_region` uses
+    /// this to size the canvas so it doesn't paint over the
+    /// rail content (codex BLOCK: `no-selection VariablesPanel
+    /// is painted under the canvas` — without this gate the
+    /// canvas extended full-width when nothing was selected,
+    /// hiding the Variables panel).
+    pub fn right_rail_visible(&self) -> bool {
+        self.property_panel_visible() || !self.var_table.variables.is_empty()
+    }
+
     /// Union of `aggregate_bounds` across selected nodes on the
     /// active page. Backs the multi-select panel's X/Y/W/H.
     pub fn selection_bounds(&self) -> Option<crate::Rect> {
diff --git a/crates/openpencil-shell-core/src/document/tests_geometry.rs b/crates/openpencil-shell-core/src/document/tests_geometry.rs
index e428befb6..7c16f7ecd 100644
--- a/crates/openpencil-shell-core/src/document/tests_geometry.rs
+++ b/crates/openpencil-shell-core/src/document/tests_geometry.rs
@@ -238,6 +238,34 @@ fn property_panel_visible_for_single_and_multi_selection() {
     assert!(doc.property_panel_visible());
 }
 
+#[test]
+fn right_rail_visible_tracks_property_panel_and_variables() {
+    use crate::document::{Variable, VariableKind, VariableScalar, VariableValue};
+    // No selection + empty var table → no right rail.
+    let mut doc = Document::empty();
+    assert!(!doc.right_rail_visible());
+
+    // Variables-only document (typical for `.op` files with themes
+    // loaded but nothing selected on first paint) — rail occupied,
+    // canvas_region must shrink so the panel isn't painted over.
+    // Codex BLOCK: canvas previously extended full-width here.
+    doc.var_table.variables.push(Variable {
+        name: "color-1".into(),
+        kind: VariableKind::Color,
+        value: VariableValue::Scalar(VariableScalar::Str("#ff0000".into())),
+    });
+    assert!(doc.right_rail_visible());
+
+    // Selection-only document → rail occupied (legacy gate path).
+    let mut doc2 = Document::sample();
+    assert!(doc2.right_rail_visible());
+
+    // Clearing both selection + vars hides the rail again.
+    doc2.clear_selection();
+    doc2.var_table.variables.clear();
+    assert!(!doc2.right_rail_visible());
+}
+
 #[test]
 fn property_panel_visible_hides_stale_single_anchor() {
     // Codex stop-hook BLOCK guard: the panel paint gate
diff --git a/crates/openpencil-shell-native/src/widget_host/geometry.rs b/crates/openpencil-shell-native/src/widget_host/geometry.rs
index 4991cc278..abaab2a74 100644
--- a/crates/openpencil-shell-native/src/widget_host/geometry.rs
+++ b/crates/openpencil-shell-native/src/widget_host/geometry.rs
@@ -257,8 +257,12 @@ impl WidgetHostNative {
         } else {
             0.0
         };
-        let has_property = self.document.property_panel_visible();
-        let canvas_right = if has_property {
+        // Any right-rail widget (PropertyPanel OR VariablesPanel)
+        // claims the rail width. Using `right_rail_visible` keeps
+        // canvas + rail mutually exclusive so the canvas never paints
+        // over an open Variables panel (codex BLOCK fix).
+        let rail_occupied = self.document.right_rail_visible();
+        let canvas_right = if rail_occupied {
             viewport_w - self.document.ui.property_panel_width
         } else {
             viewport_w