WP5.9 Customizer partially white-screens in Safari browser

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Running (Mac) Safari 13.1.2, WordPress 5.9, on multiple sites.

When loading the Customizer, I get a partial white screen – no left-column menu items (although the frame is there) and no page preview. Screenshot attached, and console error messages as follow:

[Error] Failed to load resource: the server responded with a status of 404 (Not Found) (gutenberg.css, line 0)
[Error] TypeError: o.addEventListener is not a function. (In 'o.addEventListener("change",function(e){r=e.matches})', 'o.addEventListener' is undefined)
	(anonymous function) (load-scripts.php:6:132)
	Global Code (load-scripts.php:6:111258)
[Error] TypeError: undefined is not an object (evaluating 'c.Panel.extend')
	(anonymous function) (customize-nav-menus.min.js:2:10976)
	Global Code (customize-nav-menus.min.js:2:45601)
[Error] TypeError: undefined is not an object (evaluating 'wp.customize.Control.extend')
	Global Code (slider-customizer.js:1:92)
[Error] TypeError: undefined is not an object (evaluating 'wp.customize.Control.extend')
	Global Code (slider-control.js:1:86)
[Error] TypeError: undefined is not an object (evaluating 'api.Control.extend')
	(anonymous function) (spacing-customizer.js:4)
	Global Code (spacing-customizer.js:160)
[Error] TypeError: undefined is not an object (evaluating 'api.Control.extend')
	(anonymous function) (backgrounds-customizer.js:2)
	Global Code (backgrounds-customizer.js:31)
[Error] TypeError: undefined is not an object (evaluating 'api.Control.extend')
	(anonymous function) (copyright-customizer.js:2)
	Global Code (copyright-customizer.js:12)
[Error] TypeError: undefined is not an object (evaluating 'api.Section.extend')
	(anonymous function) (upsell-control.js:2)
	Global Code (upsell-control.js:12)
[Error] TypeError: undefined is not an object (evaluating 'wp.customize.Control.extend')
	s (customizer.js:1:121358)
	(anonymous function) (customizer.js:1:131873)
	o (customizer.js:1:863)
	t (customizer.js:1:729)
	(anonymous function) (customizer.js:1:1826)
	Global Code (customizer.js:1:1829)
[Error] Error: Minified React error #200; visit https://reactjs.org/docs/error-decoder.html?invariant=200 for the full message or use the non-minified dev environment for full errors and additional helpful warnings.
	(anonymous function) (react-dom.min.js:243:307)
[Error] TypeError: The superclass is not an object.
	Oe (customize-widgets.min.js:2:28659)
	ke (customize-widgets.min.js:2:32932)
	(anonymous function) (customize.php:3105)
[Error] TypeError: undefined is not a function (near '...wp.customize.section...')
	(anonymous function) (load-scripts.php:2:31711)

Selected Element

 <div id="customize-preview" class="wp-full-overlay-main">…</div>

:Benjamin

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Screenshot of partially-loaded Customizer screen under Safari + WP5.9

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I've tried disabling all plugins and switching to Twenty Twenty-two – no change in misbehavior. Misbehavior is confined to Safari, customizer works fine under Chrome and Firefox. :B