[Feature Request] Implement min-depth support for Taxonomy Term hierarchies

Problem/Motivation

Currently, Drupal's tabledrag.js (and Gin's override) does not support a minimum depth restriction. I have developed a custom first approach to solve this, which is currently working on my site by injecting a data attribute via a preprocess function and using a modified version of Gin's tabledrag.js.

Proposed resolution

I am currently using a preprocess to add the attribute: $form['terms'][$tid]['#attributes']['data-tabledrag-min-depth'] = 1; And I have updated tabledrag.js to honor this value.

The Long-term Vision: While this manual approach works for specific use cases, the goal is to implement a standardized way to configure this within the Taxonomy UI. This feature request proposes:

Vocabulary Configuration: Adding a "Minimum Depth" setting in the Taxonomy Vocabulary settings.

Dynamic Enforcement: Using the provided JavaScript logic to ensure that the drag-and-drop interface respects these boundaries automatically.

Remaining tasks

I have already adapted Gin's tabledrag.js to support the data-tabledrag-min-depth attribute (proposed in Core issue #3004458). This ensures:

Initialization: The row object captures the minDepth.

Swap Validation: isValidSwap prevents moving a row to a position where its depth would be less than the required minimum.

Indentation Control: The indent method locks the row at the minimum level even if the user tries to drag it to the root.

Keyboard Support: Arrow keys respect the minimum depth during reordering.

API changes

I am attaching the modified tabledrag.js (based on Gin's version) which includes these checks:

// Row constructor
const minDepthAttr = $tableRow.data('tabledrag-min-depth');
if (minDepthAttr !== undefined) {
  this.minDepth = parseInt(minDepthAttr, 10);
}

// In isValidSwap
if (this.minDepth !== undefined && this.interval.max < this.minDepth) {
  return false;
}

// In indent
if (this.minDepth !== undefined && indent < this.minDepth) {
  indent = this.minDepth;
}