[RAM] flaky alerts table state test (#162805)

## Summary

closes #150790
The approach is the same as always when rendering the alerts table in
test, we mock jsdom getComputedStyle function as it has bad performance
and we don't really need that calculations

x-pack/plugins/triggers_actions_ui/public/application/sections/alerts_table/alerts_table_state.test.tsx

@@ -69,6 +69,43 @@ jest.mock('@kbn/kibana-react-plugin/public', () => ({
   }),
 }));
 
+const originalGetComputedStyle = Object.assign({}, window.getComputedStyle);
+
+beforeAll(() => {
+  // The JSDOM implementation is too slow
+  // Especially for dropdowns that try to position themselves
+  // perf issue - https://github.com/jsdom/jsdom/issues/3234
+  Object.defineProperty(window, 'getComputedStyle', {
+    value: (el: HTMLElement) => {
+      /**
+       * This is based on the jsdom implementation of getComputedStyle
+       * https://github.com/jsdom/jsdom/blob/9dae17bf0ad09042cfccd82e6a9d06d3a615d9f4/lib/jsdom/browser/Window.js#L779-L820
+       *
+       * It is missing global style parsing and will only return styles applied directly to an element.
+       * Will not return styles that are global or from emotion
+       */
+      const declaration = new CSSStyleDeclaration();
+      const { style } = el;
+
+      Array.prototype.forEach.call(style, (property: string) => {
+        declaration.setProperty(
+          property,
+          style.getPropertyValue(property),
+          style.getPropertyPriority(property)
+        );
+      });
+
+      return declaration;
+    },
+    configurable: true,
+    writable: true,
+  });
+});
+
+afterAll(() => {
+  Object.defineProperty(window, 'getComputedStyle', originalGetComputedStyle);
+});
+
 const columns = [
   {
     id: AlertsField.name,
@@ -707,8 +744,7 @@ describe('AlertsTableState', () => {
     });
   });
 
-  // FLAKY: https://github.com/elastic/kibana/issues/150790
-  describe.skip('field browser', () => {
+  describe('field browser', () => {
     const browserFields: BrowserFields = {
       kibana: {
         fields: {