Windows 10 Fall Creators Update has added Reveal highlight in the Fluent Design system. When you move your mouse on, it quickly fades out once and exposes the hidden borders around objects. As an interaction visualization, the feature is enabled by default and help users better understand the space that they are interacting with.
However, after I upgrade my Surface Pro tablet to Fall Creator’s Update, the reveal highlight is not transparency in Start menu and in the Action Center on Windows 10 1709 (OS Build 16299.19). I have already enable the Transparency and still could not get the highlight to correctly display itself, while the other apps like Calculator have reveal effect.
The Reveal is only available in a few locations in Windows 10, such as the My People hub, Action Center, and take effects with common apps, in XAML based lists and menus. For some reason, some of the new design system can’t be disabled, because they are abstract, or new part of Windows 10 or app, like perceptive parallax and conscious scrollbar. Microsoft is going to bring the Reveal effect to the Taskbar and Start menu.
If the Fluent Design effects has been disabled, the new transparency and illumination effects will be not working and that goes back to a solid color design. You could try the method to re-enable Acrylic Material and Reveal Highlight.
* Open the Settings app, go to Personalization > Colors.
* In the right side pane, move the switch of Transparency effects from Off to On.