The ContextMenu popup is not position properly relative to its target element when the page is horizontally scrolled in Edge. The problem is reproducible with R2 2018.
Steps tp reproduce:
1. Run the code below and scroll the page to the right
2. Right click over a number
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<telerik:RadScriptManager ID="RadScriptManager1" runat="server">
</telerik:RadScriptManager>
<telerik:RadContextMenu ID="rcmMenu" runat="server">
<Targets>
<telerik:ContextMenuTagNameTarget TagName="a" />
</Targets>
<Items>
<telerik:RadMenuItem Text="Menu Item 1" Value="a" />
<telerik:RadMenuItem Text="Menu Item 2" Value="b" />
</Items>
</telerik:RadContextMenu>
<div style="width: 3500px; height: 100px; position: relative; background-color: yellow;">
<a style="position: absolute; left: 1000px; top: 40px;" onclick="return showMenu(event);">1000</a>
<a style="position: absolute; left: 2000px; top: 40px;" onclick="return showMenu(event);">2000</a>
<a style="position: absolute; left: 3000px; top: 40px;" onclick="return showMenu(event);">3000</a>
</div>
</form>
</body>
</html>