fix(content): prevent editor extending on typing

[NozomuIkuta]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Resolves: #932

In addition, I adjusted some CSS styles of textarea element.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

This is UI bug, so there is nothing to change in unit tests.
Instead, I tested it in Chrome (91.0.4472.164), Firefox (90.0.2), and Safari (14.1.1) on MacBookPro (Big Sur).

Case 1: Type some characters and confirm that textarea doesn't extend on every typing.
Case 2: Type enter key several times and confirm that textarea extends by as many lines as needed.
Case 3: Delete several lines and confirm that textarea shrinks by as many lines as needed.

[NozomuIkuta]

width: 100% + horizontal padding means the element overflows parents width.
So I added box-sizing: border-box.

Default style of textarea is display: inline-block but I explicitly set it as block (maybe this is just preference).

overflow: hidden is to make situation simple and prevent redundant scroll bar from appearing.

resize: none is also to make situation simple. That is, with overflow: hidden, if developer shrinks textarea vertically, overflown lines are hidden.