use composedPath instead of target

packages/field-base/src/input-mixin.js

@@ -155,7 +155,11 @@ export const InputMixin = dedupingMixin(
  * @private
  */
  __onInput(event) {
- this._hasInputValue = event.target.value.length > 0;
+ // In the case a custom web component is passed as `inputElement`,
+ // the actual native input element, on which the event occurred,
+ // can be inside shadow trees.
+ const target = event.composedPath()[0];
+ this._hasInputValue = target.value.length > 0;
  this._onInput(event);
  }
 
@@ -166,9 +170,13 @@ export const InputMixin = dedupingMixin(
  * @protected
  */
  _onInput(event) {
+ // In the case a custom web component is passed as `inputElement`,
+ // the actual native input element, on which the event occurred,
+ // can be inside shadow trees.
+ const target = event.composedPath()[0];
  // Ignore fake input events e.g. used by clear button.
  this.__userInput = event.isTrusted;
- this.value = event.target.value;
+ this.value = target.value;
  this.__userInput = false;
  }