layout: Add very basic support for showing text in input boxes (#32365)

This only paints text in input fields. Selection and cursor are still
not painted.

In addition to adding this feature, the change also updates the
user-agent.css with the latest from the HTML specification. Extra
padding and extraneous settings (such as a bogus line-height and
min-height) are also removed from servo.css. This leads to some new
passes.

There are some new passes, this introduces failures as inserting text
reveals issues that were hidden before. Notably:

- failures in `/html/editing/editing-0/spelling-and-grammar-checking/`:
  We do not support spell-checking.
- Most of the rest of the new failures are missing features of input
  boxes that are also missing in legacy layout.

components/layout_2020/dom_traversal.rs

@@ -7,6 +7,7 @@ use std::borrow::Cow;
 use html5ever::{local_name, LocalName};
 use log::warn;
 use script_layout_interface::wrapper_traits::{ThreadSafeLayoutElement, ThreadSafeLayoutNode};
+use script_layout_interface::{LayoutElementType, LayoutNodeType};
 use servo_arc::Arc as ServoArc;
 use style::properties::ComputedValues;
 use style::selector_parser::PseudoElement;
@@ -33,7 +34,7 @@ pub(crate) struct NodeAndStyleInfo<Node> {
     pub style: ServoArc<ComputedValues>,
 }
 
-impl<Node> NodeAndStyleInfo<Node> {
+impl<'dom, Node: NodeExt<'dom>> NodeAndStyleInfo<Node> {
     fn new_with_pseudo(
         node: Node,
         pseudo_element_type: WhichPseudoElement,
@@ -53,6 +54,12 @@ impl<Node> NodeAndStyleInfo<Node> {
             style,
         }
     }
+
+    pub(crate) fn is_single_line_text_input(&self) -> bool {
+        self.node.map_or(false, |node| {
+            node.type_id() == LayoutNodeType::Element(LayoutElementType::HTMLInputElement)
+        })
+    }
 }
 
 impl<Node: Clone> NodeAndStyleInfo<Node> {
@@ -172,6 +179,23 @@ fn traverse_children_of<'dom, Node>(
         }
     }
 
+    if matches!(
+        parent_element.type_id(),
+        LayoutNodeType::Element(LayoutElementType::HTMLInputElement)
+    ) {
+        let info = NodeAndStyleInfo::new(parent_element, parent_element.style(context));
+
+        // The addition of zero-width space here forces the text input to have an inline formatting
+        // context that might otherwise be trimmed if there's no text. This is important to ensure
+        // that the input element is at least as tall as the line gap of the caret:
+        // <https://drafts.csswg.org/css-ui/#element-with-default-preferred-size>.
+        //
+        // TODO: Is there a less hacky way to do this?
+        handler.handle_text(&info, "\u{200B}".into());
+
+        handler.handle_text(&info, parent_element.to_threadsafe().node_text_content());
+    }
+
     traverse_pseudo_element(WhichPseudoElement::After, parent_element, context, handler);
 }