servo: Merge #16588 - Fix fixed table layout column width distribution (from KiChjang:fix-fixed-table-layout-column-width); r=mbrubeck

Fixes #16324.

Replaces the incorrect [CSS3 "distributing excess width to columns" algorithm](https://drafts.csswg.org/css-tables-3/#distributing-width-to-columns) and implements the simpler [CSS2 fixed table layout algorithm](https://drafts.csswg.org/css2/tables.html#fixed-table-layout).

Source-Repo: https://github.com/servo/servo
Source-Revision: bf32a7b9bd8e1fc5487596f4e9ad95550f4e021c

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 3a890b0a34931777f575cf512685200c23952608

servo/components/layout/table.rs

@@ -352,20 +352,16 @@ impl Flow for TableFlow {
 
         let shared_context = layout_context.shared_context();
         // The position was set to the containing block by the flow's parent.
+        // FIXME: The code for distributing column widths should really be placed under table_wrapper.rs.
         let containing_block_inline_size = self.block_flow.base.block_container_inline_size;
 
-        let mut num_unspecified_inline_sizes = 0;
-        let mut num_percentage_inline_sizes = 0;
-        let mut total_column_inline_size = Au(0);
-        let mut total_column_percentage_size = 0.0;
-        for column_inline_size in &self.column_intrinsic_inline_sizes {
-            if column_inline_size.percentage != 0.0 {
-                total_column_percentage_size += column_inline_size.percentage;
-                num_percentage_inline_sizes += 1;
-            } else if column_inline_size.constrained {
-                total_column_inline_size += column_inline_size.minimum_length;
-            } else {
-                num_unspecified_inline_sizes += 1;
+        let mut constrained_column_inline_sizes_indices = vec![];
+        let mut unspecified_inline_sizes_indices = vec![];
+        for (idx, column_inline_size) in self.column_intrinsic_inline_sizes.iter().enumerate() {
+            if column_inline_size.constrained {
+                constrained_column_inline_sizes_indices.push(idx);
+            } else if column_inline_size.percentage == 0.0 {
+                unspecified_inline_sizes_indices.push(idx);
             }
         }
 
@@ -383,43 +379,50 @@ impl Flow for TableFlow {
         let total_horizontal_spacing = self.total_horizontal_spacing();
         let content_inline_size = self.block_flow.fragment.border_box.size.inline -
             padding_and_borders - total_horizontal_spacing;
+        let mut remaining_inline_size = content_inline_size;
 
         match self.table_layout {
             TableLayout::Fixed => {
-                // In fixed table layout, we distribute extra space among the unspecified columns
-                // if there are any, or among all the columns if all are specified.
-                // See: https://drafts.csswg.org/css-tables-3/#distributing-width-to-columns
-                // (infobox)
                 self.column_computed_inline_sizes.clear();
-                if num_unspecified_inline_sizes != 0 {
-                    let extra_column_inline_size = content_inline_size - total_column_inline_size;
-                    for column_inline_size in &self.column_intrinsic_inline_sizes {
-                        if !column_inline_size.constrained {
-                            self.column_computed_inline_sizes.push(ColumnComputedInlineSize {
-                                size: extra_column_inline_size / num_unspecified_inline_sizes,
-                            });
-                        } else {
-                            self.column_computed_inline_sizes.push(ColumnComputedInlineSize {
-                                size: column_inline_size.minimum_length,
-                            });
-                        }
-                    }
-                } else if num_percentage_inline_sizes != 0 {
-                    let extra_column_inline_size = content_inline_size - total_column_inline_size;
-                    let ratio = content_inline_size.to_f32_px() /
-                        total_column_percentage_size;
-                    for column_inline_size in &self.column_intrinsic_inline_sizes {
+
+                // https://drafts.csswg.org/css2/tables.html#fixed-table-layout
+                for column_inline_size in &self.column_intrinsic_inline_sizes {
+                    if column_inline_size.constrained {
                         self.column_computed_inline_sizes.push(ColumnComputedInlineSize {
-                            size: extra_column_inline_size.scale_by(ratio * column_inline_size.percentage),
+                            size: column_inline_size.minimum_length,
                         });
+                        remaining_inline_size -= column_inline_size.minimum_length;
+                    } else if column_inline_size.percentage != 0.0 {
+                        let size = remaining_inline_size.scale_by(column_inline_size.percentage);
+                        self.column_computed_inline_sizes.push(ColumnComputedInlineSize {
+                            size: size,
+                        });
+                        remaining_inline_size -= size;
+                    } else {
+                        // Set the size to 0 now, distribute the remaining widths later
+                        self.column_computed_inline_sizes.push(ColumnComputedInlineSize {
+                            size: Au(0),
+                        });
+                    }
+                }
+
+                // Distribute remaining content inline size
+                if unspecified_inline_sizes_indices.len() > 0 {
+                    for &index in &unspecified_inline_sizes_indices {
+                        self.column_computed_inline_sizes[index].size =
+                            remaining_inline_size.scale_by(1.0 / unspecified_inline_sizes_indices.len() as f32);
                     }
                 } else {
-                    let ratio = content_inline_size.to_f32_px() /
-                        total_column_inline_size.to_f32_px();
-                    for column_inline_size in &self.column_intrinsic_inline_sizes {
-                        self.column_computed_inline_sizes.push(ColumnComputedInlineSize {
-                            size: column_inline_size.minimum_length.scale_by(ratio),
-                        });
+                    let total_minimum_size = self.column_intrinsic_inline_sizes
+                        .iter()
+                        .filter(|size| size.constrained)
+                        .map(|size| size.minimum_length.0 as f32)
+                        .sum::<f32>();
+
+                    for &index in &constrained_column_inline_sizes_indices {
+                        self.column_computed_inline_sizes[index].size +=
+                            remaining_inline_size.scale_by(
+                                self.column_computed_inline_sizes[index].size.0 as f32 / total_minimum_size);
                     }
                 }
             }