These shaders anti-alias the sharp pixel edges to maintain the sharpest possible image without uneven sizes (which manifests as "shimmering" during scrolling). You can try applying the same border-radius. As described by its name, box-shadow draws a shadow around the elements box, and not the elements contents, so if the box itself doesnt have rounded corners, then neither will its shadow. "pixellate" or one of the later versions with faster performance (e.g., sharp-bilinear) or more mathematically correct processing (e.g., bandlimit-pixel). Your div.navbar div.navbar-messages element lacks rounded corners, so the shadow appears square.
![retroarch shaders with rounded borderscorners retroarch shaders with rounded borderscorners](https://3.bp.blogspot.com/-5D7SJu5RGsk/VS1CStnFuuI/AAAAAAAACs8/l9GticvvnPc/s1600/lottes-flat-big.png)
The best compromise for sharp pixels without these drawbacks is to use a shader from the 'interpolation' directory, such as the o.g. Unfiltered, nearest neighbor is as sharp as you can get, but it can cause uneven pixel sizes with non-integer scale factors, which means you have to use less of your screen and/or suffer from inaccurate aspect ratios. It's all a taste thing, but as time goes on (and fewer and fewer people have actually seen these games in their original context), users are more accepting of and express a preference toward ultra-sharp pixels. Sometimes a CRT scanline shader looks even better than a LCD/Grid shader, it gives a SNES-style look that's often even better than a GBA shader. Like someone said, CRT Shaders also make GBA games look good. Though it depends on the game and the release date, and on which GBA revision hardware it was made for. The color palettes were made to be viewed through the dull screen, which is why they over-saturated them. You mentioned you like the saturated colors, but keep in mind it's usually not what the artists intended. This is a simple showcase of only the most relevant shaders that you can find in retroarch up to this date, via the online updater option, where i show you w.Buttons should be responsive to touch/mouse clicks, but was intended to be used as an decorative/cosmetic overlay/border. The best RetroArch upscaling shader is the scale of x double A fast filter, which makes the games graphics look more rounded and smooth. OpenEmu later updated to slang, but has an idiosyncratic shader file packaging system unless you work with your slang files manually).
![retroarch shaders with rounded borderscorners retroarch shaders with rounded borderscorners](https://i.redd.it/4dm4974ou7571.png)
At one point my 3 main different emu apps used 4 different formats (MAME json, OpenEmu cg, RetroARch slang & glsl. Full HD 1920x1080 Resolution, 16:9 Screen Ratio. Seems like a madhouse out there of Shader Fragmentation: Format Wars. They're like scanline filters but a grid mimicking LCD handhelds. Created this idea, of an hybrid animated control with a Classic Playstation 1 console style.
![retroarch shaders with rounded borderscorners retroarch shaders with rounded borderscorners](https://i.imgur.com/wWunyw8.jpg)
Bilinear filtering makes pixel art look horrible, though many people don't realize this (usually because they're comparing it with raw unfiltered pixels, which is also wrong and bad). RetroArch Shaders are efficient graphics filters that can greatly improve the rendering of old games.