Pull Request Diff tab : Form field causes unexpected change

Issue Summary

A change of context occurs without warning when the user changes the setting of a user interface control.
Element Description:  Filter file tree" and "Search code" search options.
Description of issue: When the above buttons are activated focus shifts to the newly appeared text input field, users are not warned ahead of time. 

Tested URL

Steps to Reproduce

1. Launch the URL

Actual Results

Expected Results

RULE : 
Changing an element's value MUST NOT automatically trigger a change of context, unless the user is adequately advised ahead of time.

HOW TO FIX: 
Fix this issue by using ONE of the following techniques:
1. Ensure that entering text into an edit box, checking a checkbox or radio button, or navigating to one of the options of a select control does not result in a substantial change in context including:
1) submitting a form automatically;
2) launching a new window;
3) an additional change of keyboard focus or
4) any other change that could confuse or disorient the user.
2. Inform users ahead of time of such behavior by methods such as the text label for the UI control or some advisory text placed before the control that cautions the user of this behavior.

BACKGROUND: 
People expect that selecting a link or a button may cause a change of context - such as a opening a new window or page, moving focus to another element, a submitting a form. Whenever the mere act of entering text into an edit box, checking a checkbox or radio button, or navigating to an option in a select control causes an unexpected change in context, some users - especially people with vision or cognitive disabilities - may become quite disoriented or confused. Changes in context should only occur when selecting a link or button - not when changing the setting of a checkbox, radio button or select - unless the user is informed ahead of time. This will ensure that users can interact with a page in a predictable manner.

Workaround

Currently, there is no known workaround for this behaviour. A workaround will be added here when available

Bug Ref: 737330