Fixes #65884

Also fixes a minor bug where setting the scroll position was still delayed even when there was no animation due to reduced motion settings.

What?

Maintain the visible content of the editor canvas when zooming in/out.

Why?

It's disorienting to zoom in/out and lose your frame of reference.

How?

When changing scale (zoom in/out):

• get the current scroll height, position, and height of the canvas
• change the canvas to fixed position to remove the scrollbar so we can smoothly animate the scale change
• reapply the scroll position, calculated off of changes to scale and frame size

It will always attempt to scale to the center unless we are at the top of the canvas. In which case, it will scale to keep the top of the canvas visible.

Known bugs

• The scaling doesn't always go to the exact center, but at least keeps your central content visible. This is due to there being changes from zoom in/out canvas sizes from appenders being added/removed, and possibly other unintended changes to content height when switching modes.
• Sometimes flickering content when changing modes: This is likely due to appenders being added/removed when the mode changes.
• Images without fixed heights such as those coming from just a URL can also cause layout shift when switching modes leading to incorrect zoom positioning.

Testing Instructions

Zooming from Top

• Toggle zoom state with the scroll bar at the top position
• You should always go to the top position (0 scrollTop)
• From zoomed out with frame space above the canvas, toggle zoom state. This should also set to 0 scroll top. It should set to 0 scroll top unless you have scrolled down past the top edge of the canvas, at which point it will center your zoom.

Zooming from below Top

• Toggle zoom state at any point beneath the top edge of the canvas
• You should be zoomed to the central point (or nearby, as the known bugs contribute to discrepancies)

Open/close sidebar scaling animations

• When zoomed out, toggle sidebars open and closed
• Animations should be the same as on wp/6.7 branch

Try to break it :)

Testing Instructions for Keyboard

Screenshots or screencast

Before

After