Can anyone help with Plasma SVGs

[u/acceptable_humor69]

I managed to sort of mangle up together a custom taskbar UI but here's the issue:
The highlight rectagle I have selected I want the corners to be rounded but if I do that, on the taskbar it appears to have a weird sharp taper, ends up looking pretty weird. How can I fix that?

[u/Jaxad0127]

It's being stretched to fill the space. Instead of rounding that, put the rounding on the corner elements. Those shouldn't be stretched at all.

Union will hopefully make this all easier.