Inconsistent look of image upload box with drag & drop location

[LinHuiqing]

Describe the bug

Inconsistent look of image upload box with drag & drop location, specifically in 2 situations:

1. I can upload when the box does not have solid line (and dragged area is in the box)
2. There’s an thin strip outside where when I drag stuff to it, there will be solid line and I can upload too

To Reproduce

Steps to reproduce the behavior:

1. Login
2. Click 'Add post'
3. In the answer field, click the image upload icon
4. Try to drag and drop an image into the image upload box

Note: image upload can only be done on desktops

Expected behavior

1. Image upload box should only be outlined when image is within the box
2. Dropped image should only be uploaded when within the box and image upload box is outlined

Screenshots
If applicable, add screenshots to help explain your problem.

Screen.Recording.2021-11-29.at.11.01.27.AM.mov

Desktop (please complete the following information):

• OS: iOS
• Chrome
• Version 96.0.4664.55 (Official Build) (x86_64)

Additional context

Bug found in PR #875