Minimal resizer

[himalinig]

I've partially resolved creating a resizer for the code editor. I need some help to fully resolve it:

1. resolving mouseDown interactions during resizing with other page elements
2. code review

Summary of changes

1. deleted all the flex-grow css. It was interfering with the resizer. Also unsure of what it was doing.
2. added resizer class to the editor div
3. made some changes to a couple css classes

Here's a video doing some visual testing on the resizer:
https://drive.google.com/file/d/1Wbf5FYTV5y3EfMnIhNeZcb3Cjb1VXWag/view?usp=sharing

Issues I need some help resolving

The video helps illustrate some of the current issues

1. there's some mouse down handler for the visualizer div that gets triggered when the cursor moves over it while resizing. Need to figure out where that handler is and make sure it only triggers when the mouse down starts in the visualizer div
2. for some reason the div kind of jumps when resizing
3. UI problem: resizer is not active over the entire div border. Just the bottom corner. Kind of easy to miss. This seems like it may not be resolvable because it's inherent to that css attribute. Might need to do further investigating and scrap this altogether