Scrollback cleared when in `--watch` mode

[s-nel]

In #2267 a change was made to clear scrollback when in --watch mode. This has degraded my development experience significantly. Before when I was in watch mode, I would to scroll back to look at compiler errors or test failures while I'm simultaneously making changes. Now I have to stop everything and wait until the next compile or test run finishes which is much slower.

As an example, say that I have 100 compile errors to fix. In my IDE I fix the errors at the bottom of the Bloop output, then when I switch back to Bloop, the IDE automatically saves which kicks off a new Bloop compile and clears the scrollback. I can't keep fixing other compiler errors because the scrollback has been cleared and I have to wait.

Another example: I have a test that takes 5 minutes to execute. It just finished executing and I have some failed tests. I accidentally change a comment in the code and save. The test starts running again, and I can't look at the previous failures until the test finishes again. Much time wasted.

Another example: hours ago I got some exception and made some changes to address it. Now I want to paste the stack trace in an issue. Previously I would find it in the scrollback, but now it's gone and I would have to make changes to reproduce it.

cc @kpodsiad