Theme · Platform: caption buttons of their own height, titles that give way

Windows caption buttons take the bar's height less its hairline, a new
token, instead of their row's, so they keep their height beside caps
that wrap below them. A title bar's title is an Ellipsis: on Windows it
takes the free width before the buttons and gives way; a centered title
sits inset past the lights on both sides. The controls never shrink.
Tests hold a long Windows title's close button inside a 280px bar and
wrapped caps inside their row.

src/theme/tokens.rs

@@ -230,6 +230,11 @@ impl Theme {
         px_to_rems(46.0)
     }
 
+    /// A Windows caption button's height: the title bar's, less its bottom hairline.
+    pub fn caption_button_height(&self) -> Pixels {
+        self.titlebar_height().to_pixels(self.base_rem()) - px(1.0)
+    }
+
     /// Resize margin of a client-drawn window.
     pub fn window_inset(&self) -> Rems {
         px_to_rems(10.0)