layout: Add support for table captions (#32657)

This adds initial support for table captions. To do this, the idea of
the table wrapper becomes a bit more concrete. Even so, the wrapper is
still reponsible for allocating space for the grid's border and padding,
as those properties are specified on the wrapper and not grid in CSS.

In order to account for this weirdness of HTML/CSS captions and grid are
now laid out and placed with a negative offset in the table wrapper
content rect.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/table/mod.rs

@@ -86,10 +86,24 @@ pub type TableSize = Size2D<usize, UnknownUnit>;
 
 #[derive(Debug, Serialize)]
 pub struct Table {
-    /// The style of this table.
+    /// The style of this table. These are the properties that apply to the "wrapper" ie the element
+    /// that contains both the grid and the captions. Not all properties are actually used on the
+    /// wrapper though, such as background and borders, which apply to the grid.
     #[serde(skip_serializing)]
     style: Arc<ComputedValues>,
 
+    /// The style of this table's grid. This is an anonymous style based on the table's style, but
+    /// eliminating all the properties handled by the "wrapper."
+    #[serde(skip_serializing)]
+    grid_style: Arc<ComputedValues>,
+
+    /// The [`BaseFragmentInfo`] for this table's grid. This is necessary so that when the
+    /// grid has a background image, it can be associated with the table's node.
+    grid_base_fragment_info: BaseFragmentInfo,
+
+    /// The captions for this table.
+    pub captions: Vec<TableCaption>,
+
     /// The column groups for this table.
     pub column_groups: Vec<TableTrackGroup>,
 
@@ -114,9 +128,16 @@ pub struct Table {
 }
 
 impl Table {
-    pub(crate) fn new(style: Arc<ComputedValues>) -> Self {
+    pub(crate) fn new(
+        style: Arc<ComputedValues>,
+        grid_style: Arc<ComputedValues>,
+        base_fragment_info: BaseFragmentInfo,
+    ) -> Self {
         Self {
             style,
+            grid_style,
+            grid_base_fragment_info: base_fragment_info,
+            captions: Vec::new(),
             column_groups: Vec::new(),
             columns: Vec::new(),
             row_groups: Vec::new(),
@@ -291,3 +312,16 @@ impl TableTrackGroup {
         self.track_range.is_empty()
     }
 }
+
+#[derive(Debug, Serialize)]
+pub struct TableCaption {
+    /// The contents of this cell, with its own layout.
+    contents: BlockFormattingContext,
+
+    /// The style of this table cell.
+    #[serde(skip_serializing)]
+    style: Arc<ComputedValues>,
+
+    /// The [`BaseFragmentInfo`] of this cell.
+    base_fragment_info: BaseFragmentInfo,
+}