Remove all IE JS fallbacks and workarounds

src/components/combo_box/__snapshots__/combo_box.test.tsx.snap

@@ -61,6 +61,7 @@ exports[`EuiComboBox is rendered 1`] = `
 exports[`props aria-label attribute is rendered 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -94,6 +95,7 @@ exports[`props aria-label attribute is rendered 1`] = `
 exports[`props aria-labelledby attribute is rendered 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -127,6 +129,7 @@ exports[`props aria-labelledby attribute is rendered 1`] = `
 exports[`props autoFocus is rendered 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -168,6 +171,7 @@ exports[`props autoFocus is rendered 1`] = `
 exports[`props custom ID is rendered 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -209,6 +213,7 @@ exports[`props custom ID is rendered 1`] = `
 exports[`props delimiter is rendered 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -250,6 +255,7 @@ exports[`props delimiter is rendered 1`] = `
 exports[`props full width is rendered 1`] = `
 <div
   className="euiComboBox euiComboBox--fullWidth"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -288,6 +294,7 @@ exports[`props full width is rendered 1`] = `
 exports[`props isClearable=false disallows user from clearing input when no options are selected 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -319,6 +326,7 @@ exports[`props isClearable=false disallows user from clearing input when no opti
 exports[`props isClearable=false disallows user from clearing input when options are selected 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -359,6 +367,7 @@ exports[`props isClearable=false disallows user from clearing input when options
 exports[`props isDisabled is rendered 1`] = `
 <div
   className="euiComboBox euiComboBox-isDisabled"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -722,6 +731,7 @@ exports[`props options list is rendered 1`] = `
 exports[`props selectedOptions are rendered 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -763,6 +773,7 @@ exports[`props selectedOptions are rendered 1`] = `
 exports[`props singleSelection is rendered 1`] = `
 <div
   className="euiComboBox"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -801,6 +812,7 @@ exports[`props singleSelection is rendered 1`] = `
 exports[`props singleSelection prepend and append is rendered 1`] = `
 <div
   className="euiComboBox euiComboBox--prepended euiComboBox--appended euiComboBox-isOpen"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput
@@ -843,6 +855,7 @@ exports[`props singleSelection prepend and append is rendered 1`] = `
 exports[`props singleSelection selects existing option when opened 1`] = `
 <div
   className="euiComboBox euiComboBox-isOpen"
+  onBlur={[Function]}
   onKeyDown={[Function]}
 >
   <EuiComboBoxInput

src/components/combo_box/combo_box.tsx

@@ -229,22 +229,9 @@ export class EuiComboBox<T> extends Component<
   // Refs
   comboBoxRefInstance: RefInstance<HTMLDivElement> = null;
   comboBoxRefCallback: RefCallback<HTMLDivElement> = (ref) => {
-    // IE11 doesn't support the `relatedTarget` event property for blur events
-    // but does add it for focusout. React doesn't support `onFocusOut` so here we are.
-    if (this.comboBoxRefInstance) {
-      this.comboBoxRefInstance.removeEventListener(
-        'focusout',
-        this.onContainerBlur
-      );
-    }
-
     this.comboBoxRefInstance = ref;
 
     if (this.comboBoxRefInstance) {
-      this.comboBoxRefInstance.addEventListener(
-        'focusout',
-        this.onContainerBlur
-      );
       const comboBoxBounds = this.comboBoxRefInstance.getBoundingClientRect();
       this.setState({
         width: comboBoxBounds.width,
@@ -549,18 +536,9 @@ export class EuiComboBox<T> extends Component<
     }
   };
 
-  onContainerBlur: EventListener = (event) => {
+  onContainerBlur: FocusEventHandler<HTMLDivElement> = (event) => {
     // close the options list, unless the user clicked on an option
-
-    /**
-     * FireFox returns `relatedTarget` as `null` for security reasons, but provides a proprietary `explicitOriginalTarget`.
-     * @see https://developer.mozilla.org/en-US/docs/Web/API/Event/explicitOriginalTarget
-     */
-    const focusEvent = event as FocusEvent & {
-      explicitOriginalTarget: EventTarget;
-    };
-    const relatedTarget = (focusEvent.relatedTarget ||
-      focusEvent.explicitOriginalTarget) as Node | null;
+    const { relatedTarget } = event;
 
     const focusedInOptionsList =
       relatedTarget &&
@@ -575,9 +553,7 @@ export class EuiComboBox<T> extends Component<
       this.closeList();
 
       if (this.props.onBlur) {
-        this.props.onBlur(
-          (event as unknown) as React.FocusEvent<HTMLDivElement>
-        );
+        this.props.onBlur(event);
       }
       this.setState({ hasFocus: false });
 
@@ -1046,6 +1022,7 @@ export class EuiComboBox<T> extends Component<
         className={classes}
         data-test-subj={dataTestSubj}
         onKeyDown={this.onKeyDown}
+        onBlur={this.onContainerBlur}
         ref={this.comboBoxRefCallback}
       >
         <EuiComboBoxInput

src/components/form/field_search/field_search.tsx

@@ -141,23 +141,14 @@ export class EuiFieldSearch extends Component<
       nativeInputValueSetter.call(this.inputElement, '');
     }
 
-    // dispatch input event, with IE11 support/fallback
-    let event;
-    if ('Event' in window && typeof Event === 'function') {
-      event = new Event('input', {
-        bubbles: true,
-        cancelable: false,
-      });
-    } else {
-      // IE11
-      event = document.createEvent('Event');
-      event.initEvent('input', true, false);
-    }
+    // dispatch input event
+    const event = new Event('input', {
+      bubbles: true,
+      cancelable: false,
+    });
 
     if (this.inputElement) {
-      if (event) {
-        this.inputElement.dispatchEvent(event);
-      }
+      this.inputElement.dispatchEvent(event);
       // set focus on the search field
       this.inputElement.focus();
       this.inputElement.dispatchEvent(new Event('change'));

src/components/form/range/range_input.tsx

@@ -35,7 +35,7 @@ export const EuiRangeInput: FunctionComponent<EuiRangeInputProps> = ({
   autoSize = true,
   ...rest
 }) => {
-  // Chrome will properly size the input based on the max value, but FF & IE do not.
+  // Chrome will properly size the input based on the max value, but FF does not.
   // Calculate the width of the input based on highest number of characters.
   // Add 2 to accommodate for input stepper
   const widthStyle = autoSize