Improve editor scrolling

[iseulde]

Maybe we can look into keeping the main toolbar hidden when scrolling down, while showing it when scrolling up. This way we can also use CSS to make showing and hiding of it smoother, instead of JavaScript. This kind of experience is widely used now, so a lot of users would already be used to it.

[iseulde]

In 37596:

Editor: better resizing for text editor

• Use scrollHeight to resize the text editor.
• Don't scroll the cursor into view when under the text editor's toolbar, it involves cloning the content which is quite slow.

Props iseulde, azaozz.

See #36482.

[iseulde]

In 36482.6.patch:

• Fix reducing.
• Don't reduce more than the minimum height.
• Scroll up the difference so that the cursor does not disappear under the toolbar when using BACKSPACE.

[azaozz]

In 36482.10.patch:

• Don't adjust the Text editor height to smaller than autoresizeMinHeight.
• Fix the adjustment on backspace.

[azaozz]

In 36482.11.patch: when shrinking the textarea, run adjust() only if it was actually resized.

[iseulde]

In 37603:

Editor: autoresize fixes for text

• Resize on backspace.
• Don't make it smaller than the minimum height.

Props azaozz, iseulde.

See #36482.

[ocean90]

@iseulde, @azaozz: What's missing here so the ticket can be closed?

[iseulde]

@ocean90 everything :) We only just improved the resizing for text.

[ocean90]

@iseulde, @azaozz: What's the plan here? Is there a follow-up commit which we can get into beta 1 tomorrow? I'm sure that the visual editor requires more testing.

[ocean90]

Closing as fixed based on ​https://wordpress.slack.com/archives/core-editor/p1467219434000232.