The Manage all template parts screen is too narrow

[carolinan]

Description

On Gutenberg trunk, there is currently a problem with the Manage all template parts screen. It is too narrow. It no longer extends to the browser's width minus the navigation sidebar.

May be related to the display flex here:

.edit-site-layout__canvas {
	align-items: center;
	bottom: 0;
	display: flex;
	justify-content: center;
	left: 0;
	position: absolute;
	top: 0;
	width: 100%;
}

Step-by-step reproduction instructions

Go to Appearance > Editor, Template parts > Manage all template parts.

Screenshots, screen recording, code snippet

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[t-hamano]

I used git bisect to locate the commit where this problem occurred. It is probably affected by #49910.