Show the placeholder text for textarea elements.

Fixes https://github.com/servo/servo/issues/10552.

All this logic was taken from htmlinputelement.rs.

components/script/dom/htmltextareaelement.rs

@@ -39,6 +39,7 @@ pub struct HTMLTextAreaElement {
     htmlelement: HTMLElement,
     #[ignore_heap_size_of = "#7193"]
     textinput: DOMRefCell<TextInput<IpcSender<ConstellationMsg>>>,
+    placeholder: DOMRefCell<DOMString>,
     // https://html.spec.whatwg.org/multipage/#concept-textarea-dirty
     value_changed: Cell<bool>,
 }
@@ -58,7 +59,12 @@ impl LayoutHTMLTextAreaElementHelpers for LayoutJS<HTMLTextAreaElement> {
     #[allow(unrooted_must_root)]
     #[allow(unsafe_code)]
     unsafe fn get_value_for_layout(self) -> String {
-        String::from((*self.unsafe_get()).textinput.borrow_for_layout().get_content())
+        let text = (*self.unsafe_get()).textinput.borrow_for_layout().get_content();
+        String::from(if text.is_empty() {
+            (*self.unsafe_get()).placeholder.borrow_for_layout().clone()
+        } else {
+            text
+        })
     }
 
     #[allow(unrooted_must_root)]
@@ -105,6 +111,7 @@ impl HTMLTextAreaElement {
             htmlelement:
                 HTMLElement::new_inherited_with_state(IN_ENABLED_STATE | IN_READ_WRITE_STATE,
                                                       local_name, prefix, document),
+            placeholder: DOMRefCell::new(DOMString::new()),
             textinput: DOMRefCell::new(TextInput::new(
                     Lines::Multiple, DOMString::new(), chan, None, None, SelectionDirection::None)),
             value_changed: Cell::new(false),
@@ -119,6 +126,14 @@ impl HTMLTextAreaElement {
                            document,
                            HTMLTextAreaElementBinding::Wrap)
     }
+
+    fn update_placeholder_shown_state(&self) {
+        let has_placeholder = !self.placeholder.borrow().is_empty();
+        let has_value = !self.textinput.borrow().is_empty();
+        let el = self.upcast::<Element>();
+        el.set_placeholder_shown_state(has_placeholder && !has_value);
+        el.set_placeholder_shown_state(has_placeholder);
+    }
 }
 
 impl HTMLTextAreaElementMethods for HTMLTextAreaElement {
@@ -311,6 +326,16 @@ impl VirtualMethods for HTMLTextAreaElement {
                     }
                 }
             },
+            local_name!("placeholder") => {
+                {
+                    let mut placeholder = self.placeholder.borrow_mut();
+                    placeholder.clear();
+                    if let AttributeMutation::Set(_) = mutation {
+                        placeholder.push_str(&attr.value());
+                    }
+                }
+                self.update_placeholder_shown_state();
+            },
             local_name!("readonly") => {
                 let el = self.upcast::<Element>();
                 match mutation {
@@ -375,10 +400,14 @@ impl VirtualMethods for HTMLTextAreaElement {
             document_from_node(self).request_focus(self.upcast());
         } else if event.type_() == atom!("keydown") && !event.DefaultPrevented() {
             if let Some(kevent) = event.downcast::<KeyboardEvent>() {
-                match self.textinput.borrow_mut().handle_keydown(kevent) {
+                // This can't be inlined, as holding on to textinput.borrow_mut()
+                // during self.implicit_submission will cause a panic.
+                let action = self.textinput.borrow_mut().handle_keydown(kevent);
+                match action {
                     KeyReaction::TriggerDefaultAction => (),
                     KeyReaction::DispatchInput => {
                         self.value_changed.set(true);
+                        self.update_placeholder_shown_state();
 
                         if event.IsTrusted() {
                             let window = window_from_node(self);

servo-tidy.toml

@@ -29,6 +29,7 @@ files = [
   "./tests/unit/net/parsable_mime/text",
   "./tests/wpt/mozilla/tests/css/fonts",
   "./tests/wpt/mozilla/tests/css/pre_with_tab.html",
+  "./tests/wpt/mozilla/tests/mozilla/textarea_placeholder.html",
   # FIXME(pcwalton, #11679): This is a workaround for a tidy error on the quoted string
   # `"__TEXT,_info_plist"` inside an attribute.
   "./components/servo/platform/macos/mod.rs",

tests/wpt/mozilla/meta/MANIFEST.json

@@ -6498,6 +6498,18 @@
             "url": "/_mozilla/mozilla/table_valign_uneven_height.html"
           }
         ],
+        "mozilla/textarea_placeholder.html": [
+          {
+            "path": "mozilla/textarea_placeholder.html",
+            "references": [
+              [
+                "/_mozilla/mozilla/textarea_placeholder_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/mozilla/textarea_placeholder.html"
+          }
+        ],
         "mozilla/webgl/clearcolor.html": [
           {
             "path": "mozilla/webgl/clearcolor.html",
@@ -21546,6 +21558,18 @@
           "url": "/_mozilla/mozilla/table_valign_uneven_height.html"
         }
       ],
+      "mozilla/textarea_placeholder.html": [
+        {
+          "path": "mozilla/textarea_placeholder.html",
+          "references": [
+            [
+              "/_mozilla/mozilla/textarea_placeholder_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/mozilla/textarea_placeholder.html"
+        }
+      ],
       "mozilla/webgl/clearcolor.html": [
         {
           "path": "mozilla/webgl/clearcolor.html",

tests/wpt/mozilla/tests/mozilla/textarea_placeholder.html

@@ -0,0 +1,11 @@
+<!doctype html>
+<meta charset="utf-8">
+<link rel="match" href="textarea_placeholder_ref.html">
+
+<textarea placeholder=""></textarea>
+<textarea placeholder="    "></textarea>
+<textarea placeholder="foobar"></textarea>
+<textarea placeholder="  foo  bar  "></textarea>
+<textarea rows=5 placeholder=" 
 foo
+ bar 
"></textarea>
+<textarea placeholder="foo bar">lorem ipsum</textarea>

tests/wpt/mozilla/tests/mozilla/textarea_placeholder_ref.html

@@ -0,0 +1,13 @@
+<!doctype html>
+<meta charset="utf-8">
+
+<textarea></textarea>
+<textarea></textarea>
+<textarea>foobar</textarea>
+<textarea>  foo  bar  </textarea>
+<textarea rows=5>
+
+ foo
+ bar
+</textarea>
+<textarea>lorem ipsum</textarea>