fix(widget-string): fix cursor jumping to end of line

Closes decaporg#3578
Closes decaporg#4539

packages/netlify-cms-widget-string/src/StringControl.js

@@ -15,6 +15,31 @@ export default class StringControl extends React.Component {
     value: '',
   };
 
+  // The selection to maintain for the input element
+  _sel = 0;
+
+  // The input element ref
+  _el = null;
+
+  // NOTE: This prevents the cursor from jumping to the end of the text for
+  // nested inputs. In other words, this is not an issue on top-level text
+  // fields such as the `title` of a collection post. However, it becomes an
+  // issue on fields nested within other components, namely widgets nested
+  // within a `markdown` widget. For example, the alt text on a block image
+  // within markdown.
+  // SEE: https://github.com/netlify/netlify-cms/issues/4539
+  // SEE: https://github.com/netlify/netlify-cms/issues/3578
+  componentDidUpdate() {
+    if (this._el && this._el.selectionStart !== this._sel) {
+      this._el.setSelectionRange(this._sel, this._sel);
+    }
+  }
+
+  handleChange = e => {
+    this._sel = e.target.selectionStart;
+    this.props.onChange(e.target.value);
+  };
+
   render() {
     const {
       forID,
@@ -27,11 +52,14 @@ export default class StringControl extends React.Component {
 
     return (
       <input
+        ref={el => {
+          this._el = el;
+        }}
         type="text"
         id={forID}
         className={classNameWrapper}
         value={value || ''}
-        onChange={e => onChange(e.target.value)}
+        onChange={this.handleChange}
         onFocus={setActiveStyle}
         onBlur={setInactiveStyle}
       />