When using the old UI, if we needed to change the page title (the text that sometimes appears in the browser tab) we could do so by editing the Main.aspx file in the instance folder (manually of from a customization plugin).
With the modern UI this is no longer possible because the main page file is Main.cshtml and it is embedded in the PX.Web.UI.Frameset.dll assembly.
One way to work around this issue is to copy the Main.cshtml file in the Views\Main folder in the Acumatica instance, but this causes problems on upgrades (we would have to manually update the file contents on each upgrade).
In order to facilitate the customization of the main page (chaging the title or another modification) I think this file should be placed inside the Views\Main folder by default. You can also refer to Acumatica case 101869 for details.