add theme titlebar height and app-owned strip dragging

crates/theme/src/resolve.rs

@@ -4,6 +4,10 @@ use crate::system::SystemTheme;
 use crate::{ColorTokens, Elevations, Motion, Radius, Scrollbar, SizeTable, ThemeVariant};
 use gpui::px;
 
+/// The floor of a custom titlebar's height, in px at 100% scaling: the height
+/// macOS and Linux titlebars draw at regardless of the control scale.
+const TITLEBAR_HEIGHT_MIN_PX: f32 = 34.0;
+
 /// Derived state shades for the colors that get pressed and hovered.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct Ramps {
@@ -25,6 +29,8 @@ pub struct ResolvedTheme {
     pub elevations: Elevations,
     pub motion: Motion,
     pub scrollbar: Scrollbar,
+    /// The height the window's titlebar strips draw at, per the platform.
+    pub titlebar_height: gpui::Pixels,
     /// The platform's native root font size; the fallback theme uses 16.
     pub root_font_size: gpui::Pixels,
     pub sizes: SizeTable,
@@ -196,6 +202,11 @@ pub(crate) fn resolve(
         sizes.set_input_height(input_height);
     }
 
+    // A platform that states its titlebar height wins.
+    let titlebar_height = system
+        .titlebar_height
+        .unwrap_or_else(|| sizes.medium.control_height.max(px(TITLEBAR_HEIGHT_MIN_PX)));
+
     let mut radius = Radius::default();
     if let Some(control) = preset
         .and_then(|def| def.control_radius)
@@ -228,6 +239,7 @@ pub(crate) fn resolve(
         elevations: Elevations::for_platform(system.source, variant),
         motion: Motion::default(),
         scrollbar: Scrollbar::default(),
+        titlebar_height,
         root_font_size,
         sizes,
         accessibility,
@@ -255,6 +267,7 @@ mod tests {
             palette,
             control_radius: None,
             input_height: None,
+            titlebar_height: None,
             root_font_size: None,
             accessibility: AccessibilityPreferences::default(),
             source: SourceKind::Macos,
@@ -303,9 +316,17 @@ mod tests {
             assert_eq!(resolved.sizes.medium.dense_padding_x, px(16. * 0.75));
             assert_eq!(resolved.sizes.medium.plate_inset, px(4.));
             assert_eq!(resolved.sizes.medium.strip_height, px(32.));
+            assert_eq!(resolved.titlebar_height, px(34.));
         }
     }
 
+    #[test]
+    fn a_stated_titlebar_height_wins_over_the_control_scale() {
+        let mut stated = system(ThemeVariant::Dark, PartialPalette::default(), None);
+        stated.titlebar_height = Some(px(32.));
+        assert_eq!(resolve(&stated, None, None).titlebar_height, px(32.));
+    }
+
     #[test]
     fn a_stated_root_font_size_scales_the_size_table() {
         // macOS states 13pt; the medium control shrinks to native proportions.