All files for deployment ready and tested - further development in this repo.

static/custom/css/admin_extras.css

@@ -0,0 +1,14 @@
+.nested-inline h3 {
+    background-color: #f3f3f3;
+    border-bottom: 1px solid #ccc;
+    padding: 0.4em;
+    margin: 0;
+    font-weight: bold;
+    cursor: pointer;
+  }
+  
+  .nested-inline fieldset.module {
+    margin-bottom: 1em;
+    border: 1px solid #ccc;
+    padding: 0;
+  }

static/custom/js/inline_toggle.js

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+document.addEventListener("DOMContentLoaded", function () {
+  const inlineSections = document.querySelectorAll(".nested-inline fieldset.module");
+
+  inlineSections.forEach(section => {
+    const header = section.querySelector("h2");
+    const content = Array.from(section.children).filter(child => !child.matches("h2"));
+
+    if (header && content.length > 0) {
+      header.style.cursor = "pointer";
+      header.style.userSelect = "none";
+      header.style.background = "#f3f3f3";
+      header.style.borderBottom = "1px solid #ccc";
+      header.style.padding = "4px";
+
+      // Collapse by default
+      content.forEach(el => el.style.display = "none");
+
+      header.addEventListener("click", () => {
+        const currentlyVisible = content[0].style.display !== "none";
+        content.forEach(el => el.style.display = currentlyVisible ? "none" : "block");
+      });
+    }
+  });
+});