Issue

You may have noticed that drag and drop functionality is not working in classic experience document libraries. 

Reason

Such issue happens because of an ambiguity between SharePoint styles and ShortPoint styles.

Workaround

Step 1: Open ShortPoint Dashboard

Please open your ShortPoint Dashboard and click the Theme Builder tile:

Step 2: Apply Custom CSS

Once Theme Builder is open, Navigate into Utilities >Custom CSS and insert this code:

div#ms-dnd-dropbox { 
display: block;
}

After you publish this changes, drag and drop functionality will start working again.

Related articles: 

• Apply Custom CSS from ShortPoint Theme Builder
• Known Issue: Page Containing Calendar with Overlays Freezes in IE