LibWeb: Make input element placeholders look better

We now create a flex container inside the input element's UA shadow tree
and add the placeholder and non-placeholder text as flex items (wrapped
in elements whose style we can manipulate).

This fixes the visual glitch where the placeholder would appear below
the bounding box of the input element. It also allows us to align the
text vertically inside the input element (like we're supposed to).

In order to achieve this, I had to make two small architectural changes
to layout tree building:

- Elements can now report that they represent a given pseudo element.
  This allows us to instantiate the ::placeholder pseudo element as an
  actual DOM element inside the input element's UA shadow tree.

- We no longer create a separate layout node for the shadow root itself.
  Instead, children of the shadow root are treated as if they were
  children of the DOM element itself for the purpose of layout tree
  building.

1 files changed, 72 insertions, 8 deletions

diff --git a/Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp
index 7cc0d5ba35..2ec5e4c762 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2022, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2018-2023, Andreas Kling <kling@serenityos.org>
  * Copyright (c) 2022, Adam Hodgen <ant1441@gmail.com>
  * Copyright (c) 2022, Andrew Kaster <akaster@serenityos.org>
  *
@@ -15,6 +15,7 @@
 #include <LibWeb/DOM/Text.h>
 #include <LibWeb/HTML/BrowsingContext.h>
 #include <LibWeb/HTML/EventNames.h>
+#include <LibWeb/HTML/HTMLDivElement.h>
 #include <LibWeb/HTML/HTMLFormElement.h>
 #include <LibWeb/HTML/HTMLInputElement.h>
 #include <LibWeb/HTML/Scripting/Environments.h>
@@ -56,7 +57,10 @@ JS::ThrowCompletionOr<void> HTMLInputElement::initialize(JS::Realm& realm)
 void HTMLInputElement::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
-    visitor.visit(m_text_node.ptr());
+    visitor.visit(m_inner_text_element);
+    visitor.visit(m_text_node);
+    visitor.visit(m_placeholder_element);
+    visitor.visit(m_placeholder_text_node);
     visitor.visit(m_legacy_pre_activation_behavior_checked_element_in_group.ptr());
     visitor.visit(m_selected_files);
 }
@@ -284,6 +288,8 @@ void HTMLInputElement::did_edit_text_node(Badge<BrowsingContext>)
     m_value = value_sanitization_algorithm(m_text_node->data());
     m_dirty_value = true;
 
+    update_placeholder_visibility();
+
     // NOTE: This is a bit ad-hoc, but basically implements part of "4.10.5.5 Common event behaviors"
     //       https://html.spec.whatwg.org/multipage/input.html#common-input-element-events
     queue_an_element_task(HTML::Task::Source::UserInteraction, [this] {
@@ -342,12 +348,26 @@ WebIDL::ExceptionOr<void> HTMLInputElement::set_value(DeprecatedString value)
     // 5. If the element's value (after applying the value sanitization algorithm) is different from oldValue,
     //    and the element has a text entry cursor position, move the text entry cursor position to the end of the
     //    text control, unselecting any selected text and resetting the selection direction to "none".
-    if (m_text_node && (m_value != old_value))
+    if (m_text_node && (m_value != old_value)) {
         m_text_node->set_data(m_value);
+        update_placeholder_visibility();
+    }
 
     return {};
 }
 
+void HTMLInputElement::update_placeholder_visibility()
+{
+    if (!m_placeholder_element)
+        return;
+    auto placeholder_text = this->placeholder_value();
+    if (placeholder_text.has_value()) {
+        MUST(m_placeholder_element->style_for_bindings()->set_property(CSS::PropertyID::Display, "block"sv));
+    } else {
+        MUST(m_placeholder_element->style_for_bindings()->set_property(CSS::PropertyID::Display, "none"sv));
+    }
+}
+
 // https://html.spec.whatwg.org/multipage/input.html#the-input-element:attr-input-placeholder-3
 static bool is_allowed_to_have_placeholder(HTML::HTMLInputElement::TypeAttributeState state)
 {
@@ -389,6 +409,17 @@ Optional<DeprecatedString> HTMLInputElement::placeholder_value() const
     return placeholder;
 }
 
+class PlaceholderElement final : public HTMLDivElement {
+    JS_CELL(PlaceholderElement, HTMLDivElement);
+
+public:
+    PlaceholderElement(DOM::Document& document)
+        : HTMLDivElement(document, DOM::QualifiedName { HTML::TagNames::div, ""sv, Namespace::HTML })
+    {
+    }
+    virtual Optional<CSS::Selector::PseudoElement> pseudo_element() const override { return CSS::Selector::PseudoElement::Placeholder; }
+};
+
 void HTMLInputElement::create_shadow_tree_if_needed()
 {
     if (shadow_root_internal())
@@ -412,7 +443,27 @@ void HTMLInputElement::create_shadow_tree_if_needed()
     if (initial_value.is_null())
         initial_value = DeprecatedString::empty();
     auto element = DOM::create_element(document(), HTML::TagNames::div, Namespace::HTML).release_value_but_fixme_should_propagate_errors();
-    MUST(element->set_attribute(HTML::AttributeNames::style, "white-space: pre; padding-top: 1px; padding-bottom: 1px; padding-left: 2px; padding-right: 2px; height: 1lh;"));
+    MUST(element->set_attribute(HTML::AttributeNames::style, R"~~~(
+        display: flex;
+        height: 100%;
+        align-items: center;
+        white-space: pre;
+        border: none;
+        padding: 1px 2px;
+)~~~"));
+
+    m_placeholder_element = heap().allocate<PlaceholderElement>(realm(), document()).release_allocated_value_but_fixme_should_propagate_errors();
+    MUST(m_placeholder_element->style_for_bindings()->set_property(CSS::PropertyID::Height, "1lh"sv));
+
+    m_placeholder_text_node = heap().allocate<DOM::Text>(realm(), document(), initial_value).release_allocated_value_but_fixme_should_propagate_errors();
+    m_placeholder_text_node->set_data(attribute(HTML::AttributeNames::placeholder));
+    m_placeholder_text_node->set_owner_input_element({}, *this);
+    MUST(m_placeholder_element->append_child(*m_placeholder_text_node));
+    MUST(element->append_child(*m_placeholder_element));
+
+    m_inner_text_element = DOM::create_element(document(), HTML::TagNames::div, Namespace::HTML).release_value_but_fixme_should_propagate_errors();
+    MUST(m_inner_text_element->style_for_bindings()->set_property(CSS::PropertyID::Height, "1lh"sv));
+
     m_text_node = heap().allocate<DOM::Text>(realm(), document(), initial_value).release_allocated_value_but_fixme_should_propagate_errors();
     m_text_node->set_always_editable(m_type != TypeAttributeState::FileUpload);
     m_text_node->set_owner_input_element({}, *this);
@@ -420,7 +471,8 @@ void HTMLInputElement::create_shadow_tree_if_needed()
     if (m_type == TypeAttributeState::Password)
         m_text_node->set_is_password_input({}, true);
 
-    MUST(element->append_child(*m_text_node));
+    MUST(m_inner_text_element->append_child(*m_text_node));
+    MUST(element->append_child(*m_inner_text_element));
     MUST(shadow_root->append_child(element));
     set_shadow_root(shadow_root);
 }
@@ -446,8 +498,13 @@ void HTMLInputElement::parse_attribute(DeprecatedFlyString const& name, Deprecat
     } else if (name == HTML::AttributeNames::type) {
         m_type = parse_type_attribute(value);
     } else if (name == HTML::AttributeNames::value) {
-        if (!m_dirty_value)
+        if (!m_dirty_value) {
             m_value = value_sanitization_algorithm(value);
+            update_placeholder_visibility();
+        }
+    } else if (name == HTML::AttributeNames::placeholder) {
+        if (m_placeholder_text_node)
+            m_placeholder_text_node->set_data(value);
     }
 }
 
@@ -474,8 +531,13 @@ void HTMLInputElement::did_remove_attribute(DeprecatedFlyString const& name)
         if (!m_dirty_checkedness)
             set_checked(false, ChangeSource::Programmatic);
     } else if (name == HTML::AttributeNames::value) {
-        if (!m_dirty_value)
+        if (!m_dirty_value) {
             m_value = DeprecatedString::empty();
+            update_placeholder_visibility();
+        }
+    } else if (name == HTML::AttributeNames::placeholder) {
+        if (m_placeholder_text_node)
+            m_placeholder_text_node->set_data({});
     }
 }
 
@@ -785,8 +847,10 @@ void HTMLInputElement::reset_algorithm()
 
     // and then invoke the value sanitization algorithm, if the type attribute's current state defines one.
     m_value = value_sanitization_algorithm(m_value);
-    if (m_text_node)
+    if (m_text_node) {
         m_text_node->set_data(m_value);
+        update_placeholder_visibility();
+    }
 }
 
 void HTMLInputElement::form_associated_element_was_inserted()