I posted this issue a few weeks back via the Forums but never received resolution. I hoped your recent update of the Slide-In plugin would have fixed the issue, but it did not.
ISSUE: When slide-in setting "Message Width" is set to a custom defined width (i.e 225px), you are not updating wdsi.css correctly. The message width within wdsi.css is updated, but all other css classes specified in the DIV (id=wdsi-slide_in) are not. The result is, since the width in the css classes are set to 100% (or 80% in one class), the DIV on the page is 100% thus overlaying all existing page DIVs. This causes any URL hyperlinks within DIV (id=wdsi-slide_in) to not work, as they are not accessible due to the overlayed slide-in DIV. Obviously, this causes an issue, especially when DIV is active over menus.
I have corrected the issue myself by modifying wdsi.css and the associated classes the DIV calls (i.e. wdsi-slide, wdsi-slide-rounded, etc.). I set the width property to match the "Message Width" setting.
I do not need a specific fix for me, but suspect others have this issue. You can view my changes at two of my sites using DevTools; http://www.silkpixel.biz or http://www.silkpixel.info. If you want to re-create the issue, you can do so utilizing DevTools...once the slide-in loads (5 second delay), click on <div id="wdsi-slide_in") and modify css class .wdsi-slide - property=width from the custom px I set to 100% (your original setting). After width is set to 100%, try to click a hyperlink, which you will not be able to within the confines of the slide-in div.