Modal Popup Screen Location
Modal is located in center of the document and not the screen itself.
One can observe in the below screen-shot that modal window (Squared in Red Color) which could be evident from scroll bar, that is squared in yellow color.
Steps to Reproduce
Open any modal dialog box.
This issue appears to be Out-of-the-box (OOTB) default configuration, though can be altered by user based on his requirement.
There is masking of background content once modal dialog is launched by a separate div, as the back ground content becomes translucent grayish color.
Hence modal dialog will be at the centre of the masked div and masked div is set to cover the total scrollable area to restrict user not to click or edit the fields, which are present behind modal dialog.
Make modal dialog position fixed, by adding below code for the masked div in UserWorkForm html rule:
position :fixed !important;
left : <AsPerRequirement>px !important;
right : <>;
bottom : <>;
top : <>;