third_party/blink/web_tests/fast/scrolling/middle-click-autoscroll-latching-clicked-node.html - chromium/src - Git at Google

 <!DOCTYPE html>
 <script src="../../resources/testharness.js"></script>
 <script src="../../resources/testharnessreport.js"></script>
 <script src="../../resources/gesture-util.js"></script>

 <style>
   #container {
     width:500px;
     height:100px;
     overflow:auto;
     border:2px solid red;
     padding:0px;
   }
   #spacer {
     height:200px;
   }
 </style>

 <ol>
   <li>Middle-click inside the &lt;div&gt; with the red border below to activate autoscroll.</li>
   <li>First, move the mouse such that you scroll the &lt;div&gt; in unavailable scroll direction and then in scrollable direction.</li>
   <li>If bug repros, it won't scroll in available direction.</li>
 </ol>
 <div id="container">
   <div id="spacer"></div>
 </div>

 <script>

 window.onload = async function()
 {
     const container = document.getElementById("container");
     const leftButton = 0;
     const middleButton = 1;

     promise_test (async (t) => {
       await waitForCompositorCommit();
       await mouseClickOn(container.offsetLeft + 10, container.offsetTop + 10, middleButton);
       //Move mouse in unavailable direction first.
       await mouseMoveTo(container.offsetLeft + (container.offsetWidth/2), container.offsetTop + 10);
       //Move mouse in available direction to scroll the content.
       await mouseMoveTo(container.offsetLeft + (container.offsetWidth/2), container.offsetTop + container.offsetHeight);

       await waitFor(() => {
         return container.scrollTop === container.scrollHeight - container.clientHeight;
       }, "Failed to scroll container with middle-click autoscroll.");
     }, "Container scrolled in available direction.");
 }
 </script>
	<!DOCTYPE html>
	<script src="../../resources/testharness.js"></script>
	<script src="../../resources/testharnessreport.js"></script>
	<script src="../../resources/gesture-util.js"></script>

	<style>
	#container {
	width:500px;
	height:100px;
	overflow:auto;
	border:2px solid red;
	padding:0px;
	}
	#spacer {
	height:200px;
	}
	</style>

	<ol>
	<li>Middle-click inside the <div> with the red border below to activate autoscroll.</li>
	<li>First, move the mouse such that you scroll the <div> in unavailable scroll direction and then in scrollable direction.</li>
	<li>If bug repros, it won't scroll in available direction.</li>
	</ol>
	<div id="container">
	<div id="spacer"></div>
	</div>

	<script>

	window.onload = async function()
	{
	const container = document.getElementById("container");
	const leftButton = 0;
	const middleButton = 1;

	promise_test (async (t) => {
	await waitForCompositorCommit();
	await mouseClickOn(container.offsetLeft + 10, container.offsetTop + 10, middleButton);
	//Move mouse in unavailable direction first.
	await mouseMoveTo(container.offsetLeft + (container.offsetWidth/2), container.offsetTop + 10);
	//Move mouse in available direction to scroll the content.
	await mouseMoveTo(container.offsetLeft + (container.offsetWidth/2), container.offsetTop + container.offsetHeight);

	await waitFor(() => {
	return container.scrollTop === container.scrollHeight - container.clientHeight;
	}, "Failed to scroll container with middle-click autoscroll.");
	}, "Container scrolled in available direction.");
	}
	</script>