EDIT: For any future searchers,
Yincognito's answer works.
TL;DR: instead of making one skin, you make two of them:
[SkinsFolder]\Rounded Corners Top\Rounded Corners Top.ini
and
[SkinsFolder]\Rounded Corners Bottom\Rounded Corners Bottom.ini
I attached both of them. It should position correctly, but if you want to drag them around, remove OnRefreshAction line.
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OLD POST:
I stumbled upon this thread by pure chance as today I decided to try getting rounded corners in Windows and found some other solution (also using Rainmeter), but after I alt-tabbed my borderless game taskbar didn't show up like normal. Looking at what Yincognito said two posts ago it seems like Windows sees a window that is "fullscreen" and decides to hide taskbar. That definitely makes sense, looking at how Rainmeter widgets are rendered. Making a window that is 1px smaller than monitor resolution works fine, but obviously looks bad (would be "okay" if you cropped your monitor's width/height in it's OSD, but it's such a bodged solution even I wouldn't do that).
Making two widgets (one for top and one for bottom) doesn't fix the issue, as rainmeter renders everything on one "window", that will get stretched to accomodate every widget (so in this case, your whole screen) getting me back to square one.
How I "fixed" it? I thought of how courageous Apple is for making their newest Macbook's screen rounded only on top and did just that - rounded it only on top. I attach my brave solution that itself is a rewritten version from
this post.
How would I resolve that?
1. Somehow get two Rainmeters running at the same time, with one rendering top corners and one bottom corners. (Definitely not the best solution)
2. Write a simple C# app that would spawn two windows, one rendering top corners, and one rendering bottom corners. (I wouldn't feel particularly safe running that, as anticheats in some games could flag it)
3. Wait for someone more knowledgeable than me to think of some other solution, as I used Rainmeter for maybe 2 hours in the last 5 years.