Allow consumers to update the size prop

- currently `size` stops being respected after the first time a user resizes the flyout

- this should work for both controlled sizes `onResize` as well as semi-uncontrolled resizing (`size` never updates)

packages/eui/src/components/flyout/flyout_resizable.spec.tsx

@@ -10,7 +10,7 @@
 /// <reference types="cypress-real-events" />
 /// <reference types="../../../cypress/support" />
 
-import React from 'react';
+import React, { useState, useCallback } from 'react';
 
 import { EuiFlyoutResizable } from './flyout_resizable';
 
@@ -196,6 +196,65 @@ describe('EuiFlyoutResizable', () => {
         expect(onResize.lastCall.args).to.eql([600]);
       });
     });
+
+    it('responds to `size` prop updates after user resize', () => {
+      let onResizeCalls = 0;
+      const TestComponent = () => {
+        const [size, setSize] = useState<string | number>(400);
+        const onResize = useCallback((width: number) => {
+          setSize(width);
+          onResizeCalls++;
+        }, []);
+
+        return (
+          <EuiFlyoutResizable onClose={onClose} size={size} onResize={onResize}>
+            <button data-test-subj="resetSize" onClick={() => setSize(400)}>
+              Reset flyout size
+            </button>
+            <button data-test-subj="setSize" onClick={() => setSize('200px')}>
+              Change flyout size
+            </button>
+            <button data-test-subj="setSizeKey" onClick={() => setSize('s')}>
+              Change flyout size to a size key
+            </button>
+          </EuiFlyoutResizable>
+        );
+      };
+      cy.mount(<TestComponent />);
+      cy.get('.euiFlyout').should('have.css', 'inline-size', '400px');
+
+      // User resizing
+      cy.get('[data-test-subj="euiResizableButton"]').focus();
+      cy.realPress('ArrowLeft').then(() => {
+        onResizeCalls = 0; // Reset resize calls. Cypress is flaky here so we shouldn't directly assert on the number of calls
+      });
+      cy.get('.euiFlyout').should('have.css', 'inline-size', '410px');
+
+      // Consumer resizing
+      cy.wait(100); // Wait a tick for flyout to finish rerendering
+      cy.get('[data-test-subj="resetSize"]').realClick();
+      cy.get('.euiFlyout')
+        .should('have.css', 'inline-size', '400px')
+        .then(() => {
+          expect(onResizeCalls).to.eql(0);
+        });
+
+      cy.wait(100); // Wait a tick for flyout to finish rerendering
+      cy.get('[data-test-subj="setSize"]').realClick();
+      cy.get('.euiFlyout')
+        .should('have.css', 'inline-size', '200px')
+        .then(() => {
+          expect(onResizeCalls).to.eql(0);
+        });
+
+      cy.wait(100); // Wait a tick for flyout to finish rerendering
+      cy.get('[data-test-subj="setSizeKey"]').realClick();
+      cy.get('.euiFlyout')
+        .should('have.css', 'inline-size', '384px')
+        .then(() => {
+          expect(onResizeCalls).to.eql(0);
+        });
+    });
   });
 
   describe('push flyouts', () => {

packages/eui/src/components/flyout/flyout_resizable.tsx

@@ -65,12 +65,20 @@ export const EuiFlyoutResizable = forwardRef(
     // Must use state for the flyout ref in order for the useEffect to be correctly called after render
     const [flyoutRef, setFlyoutRef] = useState<HTMLElement | null>(null);
     const setRefs = useCombinedRefs([setFlyoutRef, ref]);
+
+    useEffect(() => {
+      if (!flyoutWidth && flyoutRef) {
+        setCallOnResize(false); // Don't call `onResize` for non-user width changes
+        setFlyoutWidth(getFlyoutMinMaxWidth(flyoutRef.offsetWidth));
+      }
+    }, [flyoutWidth, flyoutRef, getFlyoutMinMaxWidth]);
+
+    // Update flyout width when consumers pass in a new `size`
     useEffect(() => {
-      setCallOnResize(false); // Don't call `onResize` for non-user width changes
-      setFlyoutWidth(
-        flyoutRef ? getFlyoutMinMaxWidth(flyoutRef.offsetWidth) : 0
-      );
-    }, [flyoutRef, getFlyoutMinMaxWidth, size]);
+      setCallOnResize(false);
+      // Resetting flyoutWidth to 0 will trigger the above useEffect's recalculation
+      setFlyoutWidth(typeof size === 'number' ? size : 0);
+    }, [size]);
 
     // Initial numbers to calculate from, on resize drag start
     const initialWidth = useRef(0);