Bug 1812868 - Expose scrollbar-inline-size as a chrome-only environment variable. r=mstange

After the previous patches this is rather straight-forward.

Reviewed in: https://phabricator.services.mozilla.com/D168148

layout/style/GeckoBindings.cpp

@@ -305,6 +305,16 @@ bool Gecko_AnimationNameMayBeReferencedFromStyle(
   return aPresContext->AnimationManager()->AnimationMayBeReferenced(aName);
 }
 
+float Gecko_GetScrollbarInlineSize(const nsPresContext* aPc) {
+  MOZ_ASSERT(aPc);
+  AutoWriteLock guard(*sServoFFILock);  // We read some look&feel values.
+  auto overlay = aPc->UseOverlayScrollbars() ? nsITheme::Overlay::Yes
+                                             : nsITheme::Overlay::No;
+  LayoutDeviceIntCoord size =
+      aPc->Theme()->GetScrollbarSize(aPc, StyleScrollbarWidth::Auto, overlay);
+  return aPc->DevPixelsToFloatCSSPixels(size);
+}
+
 PseudoStyleType Gecko_GetImplementedPseudo(const Element* aElement) {
   return aElement->GetPseudoElementType();
 }

layout/style/GeckoBindings.h

@@ -350,6 +350,8 @@ void Gecko_NoteAnimationOnlyDirtyElement(const mozilla::dom::Element*);
 bool Gecko_AnimationNameMayBeReferencedFromStyle(const nsPresContext*,
                                                  nsAtom* name);
 
+float Gecko_GetScrollbarInlineSize(const nsPresContext*);
+
 // Incremental restyle.
 mozilla::PseudoStyleType Gecko_GetImplementedPseudo(
     const mozilla::dom::Element*);

servo/components/style/custom_properties.rs

@@ -78,6 +78,10 @@ fn get_content_preferred_color_scheme(device: &Device) -> VariableValue {
     })
 }
 
+fn get_scrollbar_inline_size(device: &Device) -> VariableValue {
+    VariableValue::pixels(device.scrollbar_inline_size().px())
+}
+
 static ENVIRONMENT_VARIABLES: [EnvironmentVariable; 4] = [
     make_variable!(atom!("safe-area-inset-top"), get_safearea_inset_top),
     make_variable!(atom!("safe-area-inset-bottom"), get_safearea_inset_bottom),
@@ -104,7 +108,7 @@ macro_rules! lnf_int_variable {
     }};
 }
 
-static CHROME_ENVIRONMENT_VARIABLES: [EnvironmentVariable; 6] = [
+static CHROME_ENVIRONMENT_VARIABLES: [EnvironmentVariable; 7] = [
     lnf_int_variable!(
         atom!("-moz-gtk-csd-titlebar-radius"),
         TitlebarRadius,
@@ -130,6 +134,10 @@ static CHROME_ENVIRONMENT_VARIABLES: [EnvironmentVariable; 6] = [
         atom!("-moz-content-preferred-color-scheme"),
         get_content_preferred_color_scheme
     ),
+    make_variable!(
+        atom!("scrollbar-inline-size"),
+        get_scrollbar_inline_size
+    ),
 ];
 
 impl CssEnvironment {

servo/components/style/gecko/media_queries.rs

@@ -199,6 +199,18 @@ impl Device {
         unsafe { bindings::Gecko_GetBaseSize(self.document(), language.as_ptr(), generic) }
     }
 
+    /// Gets the size of the scrollbar in CSS pixels.
+    pub fn scrollbar_inline_size(&self) -> Length {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            // XXX: we could have a more reasonable default perhaps.
+            None => return Length::new(0.0),
+        };
+        Length::new(unsafe {
+            bindings::Gecko_GetScrollbarInlineSize(pc)
+        })
+    }
+
     /// Queries font metrics
     pub fn query_font_metrics(
         &self,

xpcom/ds/StaticAtoms.py

@@ -1117,6 +1117,7 @@ STATIC_ATOMS = [
     Atom("scrollbarbutton", "scrollbarbutton"),
     Atom("scrollbarDownBottom", "scrollbar-down-bottom"),
     Atom("scrollbarDownTop", "scrollbar-down-top"),
+    Atom("scrollbarInlineSize", "scrollbar-inline-size"),
     Atom("scrollbarUpBottom", "scrollbar-up-bottom"),
     Atom("scrollbarUpTop", "scrollbar-up-top"),
     Atom("scrollbox", "scrollbox"),