Bug 1856755 - Implement an internal zoom: document value that forces effective...

Bug 1856755 - Implement an internal zoom: document value that forces effective zoom to 1. r=jfkthame

The name matches the value that webkit (but not blink) exposes to the
web. We don't expose it.

Filed https://github.com/w3c/csswg-drafts/issues/9435 for this.

Differential Revision: https://phabricator.services.mozilla.com/D190025

layout/style/res/scrollbars.css

@@ -22,6 +22,9 @@
     -moz-min-font-size-ratio: initial;
     -moz-box-collapse: initial;
 
+    /* We don't want zoom on our ancestors to affect our styles. */
+    zoom: document;
+
     math-depth: initial;
     /* As long as inert implies pointer-events: none as it does now, we're
      * good. */

servo/components/style/properties/cascade.rs

@@ -897,8 +897,11 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
 
     fn compute_zoom(&mut self) {
         debug_assert!(matches!(self.cascade_mode, CascadeMode::Unvisited { .. }));
-        self.context.builder.effective_zoom =
-            self.context.builder.inherited_effective_zoom() * self.context.builder.specified_zoom();
+        self.context.builder.effective_zoom = self
+            .context
+            .builder
+            .inherited_effective_zoom()
+            .compute_effective(self.context.builder.specified_zoom());
     }
 
     fn compute_writing_mode(&mut self) {

servo/components/style/properties/longhands/box.mako.rs

@@ -640,4 +640,5 @@ ${helpers.predefined_type(
     spec="Non-standard (https://github.com/atanassov/css-zoom/ is the closest)",
     gecko_pref="layout.css.zoom.enabled",
     affects="layout",
+    enabled_in="ua",
 )}

servo/components/style/values/computed/box.rs

@@ -291,20 +291,6 @@ pub type ZoomFixedPoint = FixedPoint<u16, ZOOM_FRACTION_BITS>;
 #[repr(C)]
 pub struct Zoom(ZoomFixedPoint);
 
-impl std::ops::Mul for Zoom {
-    type Output = Zoom;
-
-    fn mul(self, rhs: Self) -> Self {
-        if self == Self::ONE {
-            return rhs;
-        }
-        if rhs == Self::ONE {
-            return self;
-        }
-        Zoom(ZoomFixedPoint::from_float(self.value() * rhs.value()))
-    }
-}
-
 impl ToComputedValue for specified::Zoom {
     type ComputedValue = Zoom;
 
@@ -312,6 +298,7 @@ impl ToComputedValue for specified::Zoom {
     fn to_computed_value(&self, _: &Context) -> Self::ComputedValue {
         let n = match *self {
             Self::Normal => return Zoom::ONE,
+            Self::Document => return Zoom::DOCUMENT,
             Self::Value(ref n) => n.0.to_number().get(),
         };
         if n == 0.0 {
@@ -332,6 +319,10 @@ impl ToCss for Zoom {
     where
         W: fmt::Write,
     {
+        use std::fmt::Write;
+        if *self == Self::DOCUMENT {
+            return dest.write_str("document");
+        }
         self.value().to_css(dest)
     }
 }
@@ -356,6 +347,10 @@ impl Zoom {
         value: 1 << ZOOM_FRACTION_BITS,
     });
 
+    /// The `document` value. This can appear in the computed zoom property value, but not in the
+    /// `effective_zoom` field.
+    pub const DOCUMENT: Zoom = Zoom(ZoomFixedPoint { value: 0 });
+
     /// Returns whether we're the number 1.
     #[inline]
     pub fn is_one(self) -> bool {
@@ -368,6 +363,20 @@ impl Zoom {
         self.0.to_float()
     }
 
+    /// Computes the effective zoom for a given new zoom value in rhs.
+    pub fn compute_effective(self, specified: Self) -> Self {
+        if specified == Self::DOCUMENT {
+            return Self::ONE;
+        }
+        if self == Self::ONE {
+            return specified;
+        }
+        if specified == Self::ONE {
+            return self;
+        }
+        Zoom(self.0 * specified.0)
+    }
+
     /// Returns the zoomed value.
     #[inline]
     pub fn zoom(self, value: f32) -> f32 {

servo/components/style/values/specified/box.rs

@@ -1891,6 +1891,10 @@ impl ScrollbarGutter {
 #[allow(missing_docs)]
 pub enum Zoom {
     Normal,
+    /// An internal value that resets the effective zoom to 1. Used for scrollbar parts, which
+    /// disregard zoom. We use this name because WebKit has this value exposed to the web.
+    #[parse(condition = "ParserContext::in_ua_sheet")]
+    Document,
     Value(NonNegativeNumberOrPercentage),
 }
 

testing/web-platform/tests/css/zoom/tentative/scroll-corner-crash.html

+<style>
+* {
+  overflow: scroll scroll;
+}
+</style>
+<h4 style="zoom: 3">
+<figure>