Fix double rendering issue for partials that yield

[camertron]

What are you trying to accomplish?

Fixes #2423

What approach did you choose and why?

ViewComponent::Base retains a reference to the view context's output buffer. When component templates are evaluated, they do so inside the context of ViewComponent::Base, and thus append to the retained @output_buffer instance. However, Rails creates a new output buffer when rendering partials, and temporarily reassigns its own @output_buffer instance variable. When rendering partials inside a view component, ViewComponent::Base forwards the block on to view_context.render, but since the block was created inside ViewComponent::Base, that's where it will be evaluated when called. This means the block will append to the original @output_buffer and not to the new one Rails creates for the partial. To mitigate the issue, this PR evaluates the block inside the view context with instance_exec, which causes the block to append to the correct buffer.

[rmacklin]

LGTM

[doits]

thank you, LGTM!