fix(widget): workaround for webview rendering bug (#3997)
See https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/extension.20performance
The bug seems to go away if we collapse the extra nested spans made by `block` in to one span.
Still should do some tests to make sure this doesn't break anything else.
Minimal breaking example is:
```
import tactic.interactive_expr
example :
0+1+2+3+4+5+6+7+8+9 +
0+1+2+3+4+5+6+7+8+9 =
0+1+2+3+4+5+6+7+8+9 :=
by skip
```
Co-authored-by: Ed Ayers <EdAyers@users.noreply.github.com>
