When previewing lots of Box Files on same web page, the webpage will jump up and down

Please fill out the following template so we can reproduce and fix your issue as quickly as possible!

Environment:
- Preview version:1.7.0
- Browser name and version:chrome and firefox
- Operating system and version: Mac OS 10.11.6

Steps to reproduce the problem:
1. preview lots of box files on same webpage
2. Programmtically focus on the viewer after it loads
```
// Programmtically focus on the viewer after it loads
if (this.viewer && this.viewer.containerEl) {
this.viewer.containerEl.focus();
}
```
3. I am not sure whether it is a good experience in this situation, because it will make the screen jump

What is the expected behavior? (Screenshots can be helpful here)
**I don't want to focus on the viewer after it loads, so could you please
provide a method or param to prevent focus automatically in source code.**

What went wrong? (Screenshots can be helpful here)

Link to application or sample code:

If relevant, link to file (or attach file here):

Start by tracing where viewer.containerEl is focused after the viewer loads, using the reported Chrome or Firefox reproduction with multiple Box files on one page. Done means the focus behavior can be disabled through a documented method or parameter and the page no longer jumps when that option is used.

Written by the indexing model from the issue text.