WYSIWYG Text Paragraph with Images Entirely Failing To Save with No Error Shown to User

Just this VERY suspicious JavaScript error, received when pressing the “Source” button and also when saving:

TypeError: f[(f.length - 1)] is undefined[Learn More] ckeditor.js:879:1
z/</<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:879:1
forEach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:293:391
forEach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:293:490
forEach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:293:490
forEach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:293:490
z/<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:877:426
g
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:10:68
CKEDITOR.event.prototype</<.fire</<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:11:428
CKEDITOR.editor.prototype.fire
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:13:67
toDataFormat
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:314:217
getData
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:1145:186
setup/<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:361:1
g
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:10:68
CKEDITOR.event.prototype</<.fire</<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:11:428
CKEDITOR.editor.prototype.fire
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:13:67
getData
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:269:95
detach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:374:94
bind/<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:28:298
detach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:355:425
detach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:1145:440
CKEDITOR.editor.prototype.editable
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:375:216
CKEDITOR.editor.prototype.setMode
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:340:223
exec
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:1056:241
CKEDITOR.command/this.exec
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:200:388
execCommand
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:268:446
CKEDITOR.ui.button/<.click<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:645:417
execute
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:646:485
render/n<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:647:367
addFunction/<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:29:451
callFunction
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:30:53
onclick
https://agaric.coop/node/171/edit:1:1

That same JavaScript error occurs when saving the node, with a slightly different stack trace:

z/</<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:879:1
forEach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:293:391
forEach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:293:490
forEach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:293:490
forEach
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:293:490
z/<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:877:426
g
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:10:68
CKEDITOR.event.prototype</<.fire</<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:11:428
CKEDITOR.editor.prototype.fire
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:13:67
toDataFormat
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:314:217
getData
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:1145:186
setup/<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:361:1
g
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:10:68
CKEDITOR.event.prototype</<.fire</<
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:11:428
CKEDITOR.editor.prototype.fire
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:13:67
getData
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:269:95
w
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:264:39
destroy
https://agaric.coop/core/assets/vendor/ckeditor/ckeditor.js:267:276
detach
https://agaric.coop/sites/default/files/js/js_ZkgxG6KMDBT6Ooxj2UPMHKXv9xq4dwKvd60CRrSfC70.js:32:11
Drupal.editorDetach
https://agaric.coop/sites/default/files/js/js_aUrTxb_8RoQV2BZUbCuq0-hIQRG2whpk2j4_cj9WPOM.js:4757:37
changeTextEditor
https://agaric.coop/sites/default/files/js/js_aUrTxb_8RoQV2BZUbCuq0-hIQRG2whpk2j4_cj9WPOM.js:4612:7
onTextFormatChange
https://agaric.coop/sites/default/files/js/js_aUrTxb_8RoQV2BZUbCuq0-hIQRG2whpk2j4_cj9WPOM.js:4675:7
dispatch
https://agaric.coop/sites/default/files/js/js_aUrTxb_8RoQV2BZUbCuq0-hIQRG2whpk2j4_cj9WPOM.js:3:10264
add/q.handle
https://agaric.coop/sites/default/files/js/js_aUrTxb_8RoQV2BZUbCuq0-hIQRG2whpk2j4_cj9WPOM.js:3:8326
receiveMessage
resource://gre/actors/SelectChild.jsm:292:13

And here’s the content of the WYSIWYG textarea that results in that error:

<p>Our most recent version of the site—when we slapped (most of) the header from the first real design, by <a href="/people/dan-hakimzadeh">co-founder Dan Hakimzadeh</a> circa 2006, onto an unadorned Bootstrap CSS framework—hadn't changed in six years:<br></p><div aria-label="A screenshot of a website featuring a circle mushroom logo, the word &quot;agaric&quot; in serif font (both next to and below the logo) and the slogan &quot;We build online&quot;. image widget" role="region" data-cke-widget-id="3" data-cke-display-name="image" class="cke_widget_wrapper cke_widget_block cke_widget_image align-center" data-cke-filter="off" data-cke-widget-wrapper="1" tabindex="-1"><figure data-cke-widget-data="%7B%22hasCaption%22%3Atrue%2C%22src%22%3A%22%2Fsites%2Fdefault%2Ffiles%2Finline-images%2Fagaric-com-through-2018.png%22%2C%22alt%22%3A%22A%20screenshot%20of%20a%20website%20featuring%20a%20circle%20mushroom%20logo%2C%20the%20word%20%5C%22agaric%5C%22%20in%20serif%20font%20(both%20next%20to%20and%20below%20the%20logo)%20and%20the%20slogan%20%5C%22We%20build%20online%5C%22.%22%2C%22width%22%3A%22%22%2C%22height%22%3A%22%22%2C%22lock%22%3Atrue%2C%22align%22%3A%22center%22%2C%22classes%22%3A%7B%7D%2C%22data-entity-type%22%3A%22file%22%2C%22data-entity-uuid%22%3A%22771e8db1-c818-47f1-bc32-7d40449c9325%22%7D" data-widget="image" data-cke-widget-keep-attr="0" class="caption caption-img cke_widget_element"><span class="cke_image_resizer_wrapper"><img data-cke-saved-src="/sites/default/files/inline-images/agaric-com-through-2018.png" src="/sites/default/files/inline-images/agaric-com-through-2018.png" alt="A screenshot of a website featuring a circle mushroom logo, the word &quot;agaric&quot; in serif font (both next to and below the logo) and the slogan &quot;We build online&quot;."><span title="Click and drag to resize" class="cke_image_resizer">​</span></span><figcaption data-placeholder="Enter caption here" data-cke-display-name="caption" class="cke_widget_editable" data-cke-filter="307" data-cke-enter-mode="2" data-cke-widget-editable="caption">Agaric.com, circa 2013 through 2018.<br></figcaption></figure><span style="background: rgba(220, 220, 220, 0.5) url(&quot;https://agaric.coop/core/assets/vendor/ckeditor/plugins/widget/images/handle.png&quot;) repeat scroll 0% 0%; top: -15px; left: 0px; display: block;" class="cke_reset cke_widget_drag_handler_container"><img role="presentation" title="Click and drag to move" src="data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw==" data-cke-widget-drag-handler="1" class="cke_reset cke_widget_drag_handler" width="15" height="15"></span></div><p><br>And this is what it looked like before that:<br><br></p><div aria-label="A screenshot of a web site with visual texture, featuring a circle mushroom logo, a serif-fonted &quot;agaric&quot; title, and the slogan &quot;making stuff better&quot;. image widget" role="region" data-cke-widget-id="1" data-cke-display-name="image" class="cke_widget_wrapper cke_widget_block cke_widget_image align-center" data-cke-filter="off" data-cke-widget-wrapper="1" tabindex="-1"><figure data-cke-widget-data="%7B%22hasCaption%22%3Atrue%2C%22src%22%3A%22%2Fsites%2Fdefault%2Ffiles%2Finline-images%2FAgaric-2010-making-stuff-better.png%22%2C%22alt%22%3A%22A%20screenshot%20of%20a%20web%20site%20with%20visual%20texture%2C%20featuring%20a%20circle%20mushroom%20logo%2C%20a%20serif-fonted%20%5C%22agaric%5C%22%20title%2C%20and%20the%20slogan%20%5C%22making%20stuff%20better%5C%22.%22%2C%22width%22%3A%22%22%2C%22height%22%3A%22%22%2C%22lock%22%3Atrue%2C%22align%22%3A%22center%22%2C%22classes%22%3A%7B%7D%2C%22data-entity-type%22%3A%22file%22%2C%22data-entity-uuid%22%3A%22fe50ef98-f982-4e04-93db-bb56ddab1785%22%7D" data-widget="image" data-cke-widget-keep-attr="0" class="caption caption-img cke_widget_element"><span class="cke_image_resizer_wrapper"><img data-cke-saved-src="/sites/default/files/inline-images/Agaric-2010-making-stuff-better.png" src="/sites/default/files/inline-images/Agaric-2010-making-stuff-better.png" alt="A screenshot of a web site with visual texture, featuring a circle mushroom logo, a serif-fonted &quot;agaric&quot; title, and the slogan &quot;making stuff better&quot;."><span title="Click and drag to resize" class="cke_image_resizer">​</span></span><figcaption data-placeholder="Enter caption here" data-cke-display-name="caption" class="cke_widget_editable" data-cke-filter="307" data-cke-enter-mode="2" data-cke-widget-editable="caption">AgaricDesign.com / Agaric.com, circa 2006—2013.<br></figcaption></figure><span style="background: rgba(220, 220, 220, 0.5) url(&quot;https://agaric.coop/core/assets/vendor/ckeditor/plugins/widget/images/handle.png&quot;) repeat scroll 0% 0%; top: -15px; left: 0px; display: block;" class="cke_reset cke_widget_drag_handler_container"><img role="presentation" title="Click and drag to move" src="data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw==" data-cke-widget-drag-handler="1" class="cke_reset cke_widget_drag_handler" width="15" height="15"></span></div><p>So you can see this latest redesign is sort of a big deal for us, as not a huge amount changed in our one major redesign between 2006 and 2018.&nbsp; Although we did lose our footer in the downgrade to lazy-developer Bootstrap theme:<br></p><p><br></p><p>I still kind of miss that footer.<br></p>

Screenshot of how (some of that) looks: https://screenshots.firefox.com/WH7A2DEnps1PSTcX/agaric.coop

And here’s the configuration of the Basic text format in use (it really is basic):

langcode: en
status: true
dependencies:
  config:
    - filter.format.basic_html
  module:
    - ckeditor
_core:
  default_config_hash: AqlPmO16LvJI4D0Ih6u4GFQIzqr5OnLgAUSjcUGWk2g
format: basic_html
editor: ckeditor
settings:
  toolbar:
    rows:
      -
        -
          name: Formatting
          items:
            - Bold
            - Italic
        -
          name: Linking
          items:
            - DrupalLink
            - DrupalUnlink
        -
          name: Lists
          items:
            - BulletedList
            - NumberedList
        -
          name: Media
          items:
            - Blockquote
            - DrupalImage
        -
          name: 'Block Formatting'
          items:
            - Format
            - Styles
            -
              name: Tools
              items:
                - Source
      plugins:
        language:
          language_list: un
        stylescombo:
          styles: "p.subtitle.is-mono.is-4|Mono subtitle\r\np.title.is-5.is-spaced|Small title\r\np.subtitle.is-6|Small subtitle"
        linebreaks:
          method: force
    image_upload:
      status: true
      scheme: public
      directory: inline-images
      max_size: ''
      max_dimensions:
        width: null
        height: null