Issue title: Problems view filter box overflows header in panel
 Issue body: Two different themes shows the issue:

![image](https://user-images.githubusercontent.com/22350/81085581-ac7a8b80-8ef7-11ea-93c2-9603c271cc00.png)

![image](https://user-images.githubusercontent.com/22350/81085587-aedce580-8ef7-11ea-91e2-10470c2891e2.png)

 Comments: 
 Comment 0: Duplicate of #96726
Also created #97438 that fixes it
{'source': 'GithubIssues', 'repo_name_with_owner': 'microsoft/vscode', 'issue_id': 612701901, 'repo_stars': 72158.0}