Add a form field component for the redesign

The label moves into the border on focus and after being filled. A valid color
is applied to the label and input border. Other states like invalid can be added
later as needed.

Adapted from @ara4n's experiment into a React component with a CSS only
approach.

res/css/_components.scss

@@ -59,6 +59,7 @@
 @import "./views/elements/_DirectorySearchBox.scss";
 @import "./views/elements/_Dropdown.scss";
 @import "./views/elements/_EditableItemList.scss";
+@import "./views/elements/_Field.scss";
 @import "./views/elements/_ImageView.scss";
 @import "./views/elements/_InlineSpinner.scss";
 @import "./views/elements/_MemberEventListSummary.scss";

res/css/views/elements/_Field.scss

@@ -0,0 +1,77 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/* TODO: Consider unifying with general input styles in _dharma.scss */
+
+.mx_Field {
+    position: relative;
+    margin: 1em 0;
+}
+
+.mx_Field input {
+    font-weight: normal;
+    border-radius: 4px;
+    transition: border-color 0.25s;
+    border: 1px solid $input-border-color;
+    padding: 8px 10px;
+}
+
+.mx_Field input:focus {
+    outline: 0;
+    border-color: $input-valid-border-color;
+}
+
+.mx_Field input::placeholder {
+    transition: color 0.25s ease-in 0s;
+    color: transparent;
+}
+
+.mx_Field input:placeholder-shown:focus::placeholder {
+    transition: color 0.25s ease-in 0.1s;
+    color: $greyed-fg-color;
+}
+
+.mx_Field label {
+    transition:
+        font-size 0.25s ease-out 0.1s,
+        color 0.25s ease-out 0.1s,
+        top 0.25s ease-out 0.1s,
+        background-color 0.25s ease-out 0.1s;
+    color: $primary-fg-color;
+    background-color: transparent;
+    font-size: 14px;
+    position: absolute;
+    left: 0px;
+    top: 0px;
+    margin: 6px 8px;
+    padding: 2px;
+}
+
+.mx_Field input:focus + label,
+.mx_Field input:not(:placeholder-shown) + label {
+    transition:
+        font-size 0.25s ease-out 0s,
+        color 0.25s ease-out 0s,
+        top 0.25s ease-out 0s,
+        background-color 0.25s ease-out 0s;
+    font-size: 10px;
+    top: -14px;
+    background-color: $field-focused-label-bg-color;
+}
+
+.mx_Field input:focus + label {
+    color: $input-valid-border-color;
+}

res/themes/dharma/css/_dharma.scss

@@ -71,6 +71,9 @@ $input-darker-bg-color: rgba(193, 201, 214, 0.29);
 $input-darker-fg-color: #9fa9ba;
 $input-lighter-bg-color: #f2f5f8;
 $input-lighter-fg-color: $input-darker-fg-color;
+$input-valid-border-color: #7ac9a1;
+
+$field-focused-label-bg-color: #ffffff;
 
 $button-bg-color: #7ac9a1;
 $button-fg-color: white;
@@ -198,8 +201,8 @@ $memberstatus-placeholder-color: $roomtile-name-color;
 
 .mx_MatrixChat {
 
-    :not(.mx_textinput) > input[type=text],
-    :not(.mx_textinput) > input[type=search],
+    :not(.mx_textinput):not(.mx_Field) > input[type=text],
+    :not(.mx_textinput):not(.mx_Field) > input[type=search],
     .mx_textinput {
         display: block;
         margin: 9px;

res/themes/light/css/_base.scss

@@ -73,6 +73,9 @@ $button-bg-color: #7ac9a1;
 $button-fg-color: white;
 // apart from login forms, which have stronger border
 $strong-input-border-color: #c7c7c7;
+$input-valid-border-color: #7ac9a1;
+
+$field-focused-label-bg-color: #ffffff;
 
 // used for UserSettings EditableText
 $input-underline-color: rgba(151, 151, 151, 0.5);

src/components/views/elements/Field.js

@@ -0,0 +1,51 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from 'react';
+import PropTypes from 'prop-types';
+
+export default class Field extends React.PureComponent {
+    static propTypes = {
+        // The field's id, which binds the input and label together.
+        id: PropTypes.string.isRequired,
+        // The field's <input> type. Defaults to "text".
+        type: PropTypes.string,
+        // The field's label string.
+        label: PropTypes.string,
+        // The field's placeholder string.
+        placeholder: PropTypes.string,
+        // All other props pass through to the <input>.
+    }
+
+    render() {
+        const extraProps = Object.assign({}, this.props);
+
+        // Remove explicit props
+        delete extraProps.id;
+        delete extraProps.type;
+        delete extraProps.placeholder;
+        delete extraProps.label;
+
+        return <div className="mx_Field">
+            <input id={this.props.id}
+                type={this.props.type || "text"}
+                placeholder={this.props.placeholder}
+                {...extraProps}
+            />
+            <label htmlFor={this.props.id}>{this.props.label}</label>
+        </div>;
+    }
+}