When calling the attachment popup window in a content page, it will appear at the top of a form and not in the viewport if the top of the form has scrolled off the window.

• Modify any catalog item to include a button for attachments at the bottom, the longer the form the easier the problem is to see.
• Include that item in an ESS page.
• Scroll to the bottom of the ESS page and click the attachments button.
• The 'Add attachment' window will appear at the top of the form regardless if the top of the form is visible.

Modify your custom UI Macro adding "parent.scrollTo(0, 0);" in the "onclick" before the attachment popup button.

For example:

<button id="attachmentButton" onclick="saveCatAttachment(gel('sysparm_item_guid').value, '$[item_table]')">Add attachment</button>

Change to: 

<button id="attachmentButton" onclick="parent.scrollTo(0, 0); saveCatAttachment(gel('sysparm_item_guid').value, '$[item_table]')">Add attachment</button>