Auto merge of #17447 - emilio:root-disconnected-subtree, r=heycam,Manishearth

style: Be more strict when setting the root font size.

Before this commit, we assumed that if the element had no parent element, it was
the root of the document, which is plain false, since we can arrive there from,
let's say, getComputedStyle on a detached node.

Bug: 1374062
Reviewed-By: heycam
MozReview-Commit-ID: 65DxdzXgd0J

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components/style/properties/gecko.mako.rs

@@ -1551,7 +1551,7 @@ fn static_assert() {
 
     pub fn fixup_none_generic(&mut self, device: &Device) {
         unsafe {
-            bindings::Gecko_nsStyleFont_FixupNoneGeneric(&mut self.gecko, &*device.pres_context)
+            bindings::Gecko_nsStyleFont_FixupNoneGeneric(&mut self.gecko, device.pres_context())
         }
     }
 
@@ -1621,7 +1621,7 @@ fn static_assert() {
     }
 
     pub fn fixup_font_min_size(&mut self, device: &Device) {
-        unsafe { bindings::Gecko_nsStyleFont_FixupMinFontSize(&mut self.gecko, &*device.pres_context) }
+        unsafe { bindings::Gecko_nsStyleFont_FixupMinFontSize(&mut self.gecko, device.pres_context()) }
     }
 
     pub fn apply_unconstrained_font_size(&mut self, v: Au) {

components/style/properties/longhand/color.mako.rs

@@ -108,7 +108,7 @@
             fn to_computed_value(&self, cx: &Context) -> Self::ComputedValue {
                 unsafe {
                     Gecko_GetLookAndFeelSystemColor(*self as i32,
-                                                    &*cx.device.pres_context)
+                                                    cx.device.pres_context())
                 }
             }
 

components/style/properties/longhand/font.mako.rs

@@ -2388,9 +2388,12 @@ ${helpers.single_keyword("-moz-math-variant",
 
                 let mut system: nsFont = unsafe { mem::uninitialized() };
                 unsafe {
-                    bindings::Gecko_nsFont_InitSystem(&mut system, id as i32,
-                                            cx.style.get_font().gecko(),
-                                            &*cx.device.pres_context)
+                    bindings::Gecko_nsFont_InitSystem(
+                        &mut system,
+                        id as i32,
+                        cx.style.get_font().gecko(),
+                        cx.device.pres_context()
+                    )
                 }
                 let family = system.fontlist.mFontlist.iter().map(|font| {
                     use properties::longhands::font_family::computed_value::*;

components/style/properties/properties.mako.rs

@@ -2472,20 +2472,24 @@ bitflags! {
         /// Whether to inherit all styles from the parent. If this flag is not
         /// present, non-inherited styles are reset to their initial values.
         const INHERIT_ALL = 0x01,
+
         /// Whether to skip any root element and flex/grid item display style
         /// fixup.
         const SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP = 0x02,
+
         /// Whether to only cascade properties that are visited dependent.
         const VISITED_DEPENDENT_ONLY = 0x04,
-        /// Should we modify the device's root font size
-        /// when computing the root?
+
+        /// Whether the given element we're styling is the document element,
+        /// that is, matches :root.
         ///
-        /// Not set for native anonymous content since some NAC
-        /// form their own root, but share the device.
+        /// Not set for native anonymous content since some NAC form their own
+        /// root, but share the device.
         ///
-        /// ::backdrop and all NAC will resolve rem units against
-        /// the toplevel root element now.
-        const ALLOW_SET_ROOT_FONT_SIZE = 0x08,
+        /// This affects some style adjustments, like blockification, and means
+        /// that it may affect global state, like the Device's root font-size.
+        const IS_ROOT_ELEMENT = 0x08,
+
         /// Whether to convert display:contents into display:inline.  This
         /// is used by Gecko to prevent display:contents on generated
         /// content.
@@ -2520,15 +2524,13 @@ pub fn cascade(device: &Device,
                quirks_mode: QuirksMode)
                -> ComputedValues {
     debug_assert_eq!(parent_style.is_some(), layout_parent_style.is_some());
-    let (is_root_element, inherited_style, layout_parent_style) = match parent_style {
+    let (inherited_style, layout_parent_style) = match parent_style {
         Some(parent_style) => {
-            (false,
-             parent_style,
+            (parent_style,
              layout_parent_style.unwrap())
         },
         None => {
-            (true,
-             device.default_computed_values(),
+            (device.default_computed_values(),
              device.default_computed_values())
         }
     };
@@ -2558,7 +2560,6 @@ pub fn cascade(device: &Device,
         })
     };
     apply_declarations(device,
-                       is_root_element,
                        iter_declarations,
                        inherited_style,
                        layout_parent_style,
@@ -2574,7 +2575,6 @@ pub fn cascade(device: &Device,
 /// first.
 #[allow(unused_mut)] // conditionally compiled code for "position"
 pub fn apply_declarations<'a, F, I>(device: &Device,
-                                    is_root_element: bool,
                                     iter_declarations: F,
                                     inherited_style: &ComputedValues,
                                     layout_parent_style: &ComputedValues,
@@ -2629,7 +2629,7 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
     };
 
     let mut context = computed::Context {
-        is_root_element: is_root_element,
+        is_root_element: flags.contains(IS_ROOT_ELEMENT),
         device: device,
         inherited_style: inherited_style,
         layout_parent_style: layout_parent_style,
@@ -2783,13 +2783,14 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
                     // which Gecko just does regular cascading with. Do the same.
                     // This can only happen in the case where the language changed but the family did not
                     if generic != structs::kGenericFont_NONE {
-                        let pres_context = context.device.pres_context;
-                        let gecko_font = context.mutate_style().mutate_font().gecko_mut();
+                        let gecko_font = context.style.mutate_font().gecko_mut();
                         gecko_font.mGenericID = generic;
                         unsafe {
-                            bindings::Gecko_nsStyleFont_PrefillDefaultForGeneric(gecko_font,
-                                                                                 &*pres_context,
-                                                                                 generic);
+                            bindings::Gecko_nsStyleFont_PrefillDefaultForGeneric(
+                                gecko_font,
+                                context.device.pres_context(),
+                                generic
+                            );
                         }
                     }
                 }
@@ -2853,7 +2854,7 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
             % endif
             }
 
-            if is_root_element && flags.contains(ALLOW_SET_ROOT_FONT_SIZE) {
+            if context.is_root_element {
                 let s = context.style.get_font().clone_font_size();
                 context.device.set_root_font_size(s);
             }
@@ -2863,7 +2864,7 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
     let mut style = context.style;
 
     {
-        StyleAdjuster::new(&mut style, is_root_element)
+        StyleAdjuster::new(&mut style)
             .adjust(context.layout_parent_style, flags);
     }