TYPO3 Core

In File:

Build/Sources/TypeScript/backend/Resources/Public/TypeScript/FormEngine.ts

Line 826 -832
Condition if content was changed.

if (FormEngine.hasChange()) {
  FormEngine.showPreviewModal(previewUrl, isNew, $actionElement, callback);
} else {
  $('form[name=' + FormEngine.formName + ']').append($actionElement);
  window.open('', 'newTYPO3frontendWindow');
  ((document as any).editform as HTMLFormElement).submit();
}

Line 654 - 658
Check if content has "has-change" class.

FormEngine.hasChange = function(): boolean {
  const formElementChanges = $('form[name="' + FormEngine.formName + '"] .has-change').length > 0,
   inlineRecordChanges = $('[name^="data["].has-change').length > 0;
  return formElementChanges || inlineRecordChanges;
};

Build/Sources/TypeScript/backend/Resources/Public/TypeScript/FormEngineValidation.ts
In File has-change class is set on "form-group t3js-formengine-validation-marker t3js-formengine-palette-field has-change" for example.

Line 570 ff

  /**
   * Helper function to mark a field as changed.
   *
   * @param {HTMLInputElement|HTMLTextAreaElement|jQuery} field
   */
  FormEngineValidation.markFieldAsChanged = function(field: HTMLInputElement|HTMLTextAreaElement|HTMLSelectElement|JQuery): void {
    if (field instanceof $) {
      field = <HTMLInputElement|HTMLTextAreaElement|HTMLSelectElement>(field as JQuery).get(0);
    }
    if (!(field instanceof HTMLElement)) {
      return;
    }
    const paletteField = field.closest('.t3js-formengine-palette-field');
    paletteField.classList.add('has-change');
  };

But this class seems to be removed, when content is saved.

If I try to "View" the changes, but content isn't saved yet, I'm forced to save the content first before going to the preview tab.
Imho in this case FormEngine.hasChange is never true.

if (FormEngine.hasChange()) {
  FormEngine.showPreviewModal(previewUrl, isNew, $actionElement, callback);
}