chromium/components/browser_ui/widget/android/java/res/drawable/query_tile_overlay.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
Copyright 2020 The Chromium Authors
Use of this source code is governed by a BSD-style license that can be
found in the LICENSE file.
-->
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
  <item android:state_selected="false">
    <shape android:shape="rectangle" >
      <gradient
          android:angle="270"
          android:endColor="@color/black_alpha_65"
          android:startColor="@android:color/transparent"
          android:type="linear" />
      <corners
          android:radius="@dimen/default_rounded_corner_radius"/>
    </shape>
  </item>

  <!-- Selected state. Same as default + hairline stroke. -->
  <item android:state_selected="true">
    <shape android:shape="rectangle">
      <stroke android:width="2dp" android:color="@macro/hairline_stroke_color"/>
      <gradient
          android:angle="270"
          android:endColor="@color/black_alpha_65"
          android:startColor="@android:color/transparent"
          android:type="linear" />
      <corners
          android:radius="@dimen/default_rounded_corner_radius"/>
    </shape>
  </item>
</selector>