fix(button): centre labels on their drawn width

Button measured its label in the configured button_font family (an
SVG-inked extent) but draws it with no family, in the UI sans. Under the
default Berkeley Mono the measure ran wide ("Attach..." 73px vs 53px
drawn), so every label sat left of centre and intrinsic_size padded the
right. label_width now shapes the label the way paint draws it, through
the geometry font system, keeping the inked measure as a fallback.

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

 src/widget/input/button.rs | 53 ++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 49 insertions(+), 4 deletions(-)

diff --git a/src/widget/input/button.rs b/src/widget/input/button.rs
index d484b25..f5465b5 100644
--- a/src/widget/input/button.rs
+++ b/src/widget/input/button.rs
@@ -234,13 +234,37 @@ impl Button {
         (family, size.unwrap_or(12.0))
     }
 
+    /// The label's width AS `paint` DRAWS IT: shaped by cosmic-text with no
+    /// family (the DE's UI sans), which is what the `text_with(.., None, ..)`
+    /// below emits — the same shaped-buffer cache the draw reads.
+    ///
+    /// This used to be `measure_text_width` in the configured `button_font`
+    /// family: an SVG-inked extent in a face the label is never drawn in.
+    /// Under the default Berkeley Mono that measured every label wider than
+    /// its sans draw ("Attach..." 73px against 53px drawn), and since both
+    /// the centring and `intrinsic_size` read this, every label sat left of
+    /// centre with the surplus piled up on its right. The inked measure is
+    /// now only the fallback for a font system that shapes nothing.
+    ///
+    /// Blocking lock, as the context menu takes it: `try_lock` fell back to
+    /// the wrong measure whenever another thread was shaping. The one site
+    /// that holds this lock across a widget call (the flat host, around
+    /// `prepare_text`) never reaches a Button's measure — keep it that way.
     fn label_width(&self, label: &str) -> f32 {
         if label == "📋" {
-            12.0
-        } else {
-            let (family, size) = self.font();
-            crate::widget::display::measure_text_width(label, &family, size)
+            return 12.0;
         }
+        let (family, size) = self.font();
+        crate::geometry_font_system()
+            .lock()
+            .ok()
+            .and_then(|mut fs| {
+                crate::backend::window_runner::shaped_cluster_offsets(&mut fs, label, size, None)
+                    .last()
+                    .map(|&(_, total)| total)
+            })
+            .filter(|&w| w > 0.0)
+            .unwrap_or_else(|| crate::widget::display::measure_text_width(label, &family, size))
     }
 
     /// The control plate this Button's `paint` draws — flush, at the button
@@ -689,6 +713,26 @@ mod tests {
         assert!(x > 10.0 && x < 130.0, "a fitting label must stay centred, got {x}");
     }
 
+    /// Centred means centred on the glyphs as DRAWN: the gap either side of
+    /// the shaped label is equal. Measuring in the configured button family
+    /// while drawing in the UI sans left "Attach..." ~10px left of centre.
+    #[test]
+    fn a_label_is_centred_on_its_drawn_width() {
+        let (_, size) = Button::model(ButtonKind::Primary).font();
+        for label in ["Attach...", "Load Images", "Cancel"] {
+            let drawn = {
+                let mut fs = crate::geometry_font_system().lock().unwrap();
+                crate::backend::window_runner::shaped_cluster_offsets(&mut fs, label, size, None)
+                    .last()
+                    .map(|&(_, t)| t)
+                    .unwrap()
+            };
+            let (x, _) = painted_label(label, 160.0);
+            let (left, right) = (x - 10.0, 170.0 - (x + drawn));
+            assert!((left - right).abs() < 1.0, "{label:?}: {left:.1}px left vs {right:.1}px right");
+        }
+    }
+
     fn press(x: f32, y: f32) -> Event {
         Event::MouseButton { button: MouseButton::Left, state: ElementState::Pressed, x, y, local_x: x, local_y: y }
     }
@@ -795,3 +839,4 @@ mod focus_ring_tests {
         assert_eq!(troughs(&b), vec![None]);
     }
 }
+