Tests: add manual test.

tests/plugins/autocomplete/manual/windowborder.html

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+<style>
+	html,
+	body {
+		height: 100%;
+	}
+
+	.wrapper {
+		position: absolute;
+		bottom: 0;
+		display: inline-flex;
+	}
+
+	.wrapper > div:not(:last-of-type) {
+		margin-right: 15px;
+	}
+</style>
+
+<div class="wrapper">
+	<div id="classic">
+		<p>1 &nbsp;</p>
+		<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Expedita, veritatis.</p>
+		<p>2 &nbsp;</p>
+	</div>
+
+	<div id="divarea">
+		<p>1 &nbsp;</p>
+		<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Expedita, veritatis.</p>
+		<p>2 &nbsp;</p>
+	</div>
+
+	<div id="inline" contenteditable="true">
+		<p>1 &nbsp;</p>
+		<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Expedita, veritatis.</p>
+		<p>2 &nbsp;</p>
+	</div>
+</div>
+
+<script>
+	bender.tools.ignoreUnsupportedEnvironment( 'autocomplete' );
+
+	var config = {
+		width: 300,
+		height: 150,
+		on: {
+			instanceReady: function( evt ) {
+				new CKEDITOR.plugins.autocomplete( evt.editor, {
+					textTestCallback: autocompleteUtils.getTextTestCallback(),
+					dataCallback: autocompleteUtils.getDataCallback()
+				} );
+			}
+		}
+	};
+
+	CKEDITOR.replace( 'classic', config );
+	CKEDITOR.replace( 'divarea', CKEDITOR.tools.extend( { extraPlugins: 'divarea' }, config ) );
+	CKEDITOR.inline( 'inline', config );
+</script>

tests/plugins/autocomplete/manual/windowborder.md

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+@bender-tags: 4.16.0, feature, 3582
+@bender-ui: collapsed
+@bender-ckeditor-plugins: sourcearea, wysiwygarea, floatingspace, toolbar, autocomplete, textmatch
+@bender-include: _helpers/utils.js
+
+1. Place cursor after `2 `.
+1. Type `@`.
+
+*Repeat steps above for all 3 editors.*
+
+## Expected
+
+Mention panel should be appeared above cursor.
+
+## Unexpected
+
+Mention panel is not appearing above cursor.