I couldn't debug my Client-side Blazor, breakpoints would simple not get hit.
I also disabled the "use previews of .NET Core SDK" (Options->Environment->Preview Features), which I was using previously.
Visual Studio 16.6.2.
My .csproj had:
<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
<DebugType>full</DebugType>
...
</PropertyGroup>
Simply removed the line with DebugType and it's working as intended.