layout/
layout_root.rs

1/* This Source Code Form is subject to the terms of the Mozilla Public
2 * License, v. 2.0. If a copy of the MPL was not distributed with this
3 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
4
5use script::layout_dom::ServoLayoutNode;
6
7use crate::context::LayoutContext;
8use crate::dom::{LayoutBox, NodeExt};
9use crate::flexbox::FlexLevelBox;
10use crate::flow::BlockLevelBox;
11use crate::flow::inline::InlineItem;
12use crate::fragment_tree::Fragment;
13
14/// A data structure to track a layout root.
15///
16/// Layout roots are places in the fragment tree where fragment tree layout can start.
17/// These are fragments that isolate fragment damage from their ancestors sufficiently
18/// that their ancestors can be preserved from one fragment tree layout to the next.
19/// **Currently, this only includes absolutely-or-fixed-positioned fragments that do not
20/// have any escaping fixed position fragments.**.
21///
22/// During damage propagation, upward flowing fragment tree `Relayout` damage can be
23/// converted into `DescendantCollectedAsLayoutRoot` damage. When an ancestor of a layout
24/// root has fragment tree layout damage, `DescendantCollectedAsLayoutRoot` damage is
25/// converted back into `Relayout` and propagated up to the next layout root or the root
26/// of the DOM.
27///
28/// This is only possible because the damage propagation traversal ensures that damage
29/// above the absolute (which might shift its static position) is converted back into
30/// `Relayout` damage. This means that the absolute should be placed with the fully
31/// adjusted static positioning rectangle between two layouts.
32///
33/// It's possible that upon laying out a layout root again, a new fixed position element
34/// has started to escape from the layout root. In that case, a full fragment tree layout
35/// becomes necessary to rebuild the fragment properly.
36pub(crate) struct LayoutRoot<'dom> {
37    node: ServoLayoutNode<'dom>,
38}
39
40impl<'dom> TryFrom<ServoLayoutNode<'dom>> for LayoutRoot<'dom> {
41    type Error = ();
42
43    fn try_from(node: ServoLayoutNode<'dom>) -> Result<Self, Self::Error> {
44        if !node.is_absolutely_positioned() {
45            return Err(());
46        }
47
48        // Only accept previously known layout roots without fragmentation as compatible layout roots.
49        let mut is_layout_root = false;
50        node.with_layout_box_base(|base| {
51            let fragments = base.fragments();
52            is_layout_root =
53                fragments.len() == 1 && matches!(fragments.first(), Some(Fragment::LayoutRoot(..)));
54        });
55        if !is_layout_root {
56            return Err(());
57        }
58
59        Ok(Self { node })
60    }
61}
62
63impl LayoutRoot<'_> {
64    pub(crate) fn try_layout(&self, layout_context: &LayoutContext) -> bool {
65        let Some(inner_layout_data) = self.node.inner_layout_data_mut() else {
66            return false;
67        };
68        let layout_box = inner_layout_data.self_box.clone();
69
70        let layout_box = layout_box.borrow();
71        let Some(layout_box) = &*layout_box else {
72            return false;
73        };
74
75        let positioned_box = match layout_box {
76            LayoutBox::BlockLevel(block_level) => {
77                let mut block_level = block_level.borrow_mut();
78                match &mut *block_level {
79                    BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(positioned_box) => {
80                        positioned_box.clone()
81                    },
82                    _ => return false,
83                }
84            },
85            LayoutBox::InlineLevel(InlineItem::OutOfFlowAbsolutelyPositionedBox(
86                positioned_box,
87                _,
88            )) => positioned_box.clone(),
89            LayoutBox::FlexLevel(flex_level_box) => {
90                let mut flex_level_box = flex_level_box.borrow_mut();
91                match &mut *flex_level_box {
92                    FlexLevelBox::OutOfFlowAbsolutelyPositionedBox(positioned_box) => {
93                        positioned_box.clone()
94                    },
95                    _ => return false,
96                }
97            },
98            _ => return false,
99        };
100
101        let positioned_box = positioned_box.borrow();
102        let formatting_context = &positioned_box.context;
103        let fragments = formatting_context.base.fragments();
104        let Some(Fragment::LayoutRoot(layout_root_fragment)) = fragments.first() else {
105            return false;
106        };
107
108        let layout_inputs = formatting_context.layout_root_layout_inputs.borrow();
109        let Some(layout_inputs) = &*layout_inputs else {
110            return false;
111        };
112
113        // If an `Err` is returned here, that means that this layout root is no longer
114        // a viable layout root and a full fragment tree layout is necessary.
115        layout_inputs
116            .layout(
117                layout_context,
118                formatting_context,
119                &layout_root_fragment.fragment,
120            )
121            .is_ok()
122    }
123}
layout/layout_root.rs

layout/
layout_root.rs
