style: Make content-visibility: auto forces contain-intrinsic-size to gain an auto value

When the content having `content-visibility: auto` and the specific value for `contain-intrinsic-size` is slightly out of the viewport,
its computed value keeps changing.
This patch makes `content-visibilty: auto` forces `contain-intrinsic-size` to gain an auto value to solve this issue.

Differential Revision: https://phabricator.services.mozilla.com/D174583

components/style/style_adjuster.rs

@@ -532,6 +532,32 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
             .set_effective_containment(new_contain);
     }
 
+    /// content-visibility: auto should force contain-intrinsic-size to gain
+    /// an auto value
+    ///
+    /// <https://github.com/w3c/csswg-drafts/issues/8407>
+    fn adjust_for_contain_intrinsic_size(&mut self) {
+        let content_visibility = self.style.get_box().clone_content_visibility();
+        if content_visibility != ContentVisibility::Auto {
+            return;
+        }
+
+        let pos = self.style.get_position();
+        let new_width = pos.clone_contain_intrinsic_width().add_auto_if_needed();
+        let new_height = pos.clone_contain_intrinsic_height().add_auto_if_needed();
+        if new_width.is_none() && new_height.is_none() {
+            return;
+        }
+
+        let pos = self.style.mutate_position();
+        if let Some(width) = new_width {
+            pos.set_contain_intrinsic_width(width);
+        }
+        if let Some(height) = new_height {
+            pos.set_contain_intrinsic_height(height);
+        }
+    }
+
     /// Handles the relevant sections in:
     ///
     /// https://drafts.csswg.org/css-display/#unbox-html
@@ -952,6 +978,7 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
         self.adjust_for_webkit_line_clamp();
         self.adjust_for_position();
         self.adjust_for_overflow();
+        self.adjust_for_contain_intrinsic_size();
         #[cfg(feature = "gecko")]
         {
             self.adjust_for_contain();

components/style/values/computed/box.rs

@@ -25,6 +25,20 @@ pub type VerticalAlign = GenericVerticalAlign<LengthPercentage>;
 /// A computed value for the `contain-intrinsic-size` property.
 pub type ContainIntrinsicSize = GenericContainIntrinsicSize<NonNegativeLength>;
 
+impl ContainIntrinsicSize {
+    /// Converts contain-intrinsic-size to auto style.
+    pub fn add_auto_if_needed(&self) -> Option<Self> {
+        use crate::Zero;
+        // TODO: support contain-intrinsic-size: auto none, see
+        // https://bugzilla.mozilla.org/show_bug.cgi?id=1835813
+        Some(match *self {
+            Self::None => Self::AutoLength(Zero::zero()),
+            Self::Length(ref l) => Self::AutoLength(*l),
+            Self::AutoLength(..) => return None,
+        })
+    }
+}
+
 /// A computed value for the `line-clamp` property.
 pub type LineClamp = GenericLineClamp<Integer>;