fix(ui): blocks field not respecting width styles in row layouts (#13502)

### What?

This PR applies `mergeFieldStyles` to the `BlocksField` component,
ensuring that custom admin styles such as `width` are correctly
respected when Blocks fields are placed inside row layouts.

### Why?

Previously, Blocks fields did not inherit or apply their `admin.width`
(or other merged field styles). For example, when placing two Blocks
fields side by side inside a row with `width: '50%'`, the widths were
ignored, causing layout issues.

### How?

- Imported and used `mergeFieldStyles` within `BlocksField`.
- Applied the merged styles to the root `<div>` via the `style` prop,
consistent with how other field components (like `TextField`) handle
styles.

Fixes #13498

packages/ui/src/fields/Blocks/index.tsx

@@ -35,6 +35,7 @@ import './index.scss'
 import { FieldDescription } from '../FieldDescription/index.js'
 import { FieldError } from '../FieldError/index.js'
 import { FieldLabel } from '../FieldLabel/index.js'
+import { mergeFieldStyles } from '../mergeFieldStyles.js'
 import { fieldBaseClass } from '../shared/index.js'
 import { BlockRow } from './BlockRow.js'
 import { BlocksDrawer } from './BlocksDrawer/index.js'
@@ -45,6 +46,7 @@ const BlocksFieldComponent: BlocksFieldClientComponent = (props) => {
   const { i18n, t } = useTranslation()
 
   const {
+    field,
     field: {
       name,
       type,
@@ -294,6 +296,8 @@ const BlocksFieldComponent: BlocksFieldClientComponent = (props) => {
   const showMinRows = rows.length < minRows || (required && rows.length === 0)
   const showRequired = readOnly && rows.length === 0
 
+  const styles = useMemo(() => mergeFieldStyles(field), [field])
+
   return (
     <div
       className={[
@@ -305,6 +309,7 @@ const BlocksFieldComponent: BlocksFieldClientComponent = (props) => {
         .filter(Boolean)
         .join(' ')}
       id={`field-${path?.replace(/\./g, '__')}`}
+      style={styles}
     >
       {showError && (
         <RenderCustomComponent

test/fields/collections/Row/e2e.spec.ts

@@ -205,4 +205,41 @@ describe('Row', () => {
       expect(fieldABox.height).toEqual(fieldBBox.height)
     }).toPass()
   })
+
+  test('should respect admin.width for Blocks fields inside a row', async () => {
+    await page.goto(url.create)
+
+    // Target the Blocks field wrappers
+    const left = page.locator('#field-leftColumn')
+    const right = page.locator('#field-rightColumn')
+
+    await expect(left).toBeVisible()
+    await expect(right).toBeVisible()
+
+    // 1) CSS variable is applied (via mergeFieldStyles)
+    const leftVar = await left.evaluate((el) =>
+      getComputedStyle(el).getPropertyValue('--field-width').trim(),
+    )
+    const rightVar = await right.evaluate((el) =>
+      getComputedStyle(el).getPropertyValue('--field-width').trim(),
+    )
+
+    expect(leftVar).toBe('50%')
+    expect(rightVar).toBe('50%')
+
+    // Also assert inline style contains the var (robust to other inline styles)
+    await expect(left).toHaveAttribute('style', /--field-width:\s*50%/)
+    await expect(right).toHaveAttribute('style', /--field-width:\s*50%/)
+
+    // 2) Layout reflects the widths (same row, equal widths)
+    const leftBox = await left.boundingBox()
+    const rightBox = await right.boundingBox()
+
+    await expect(() => {
+      // Same row
+      expect(Math.round(leftBox.y)).toEqual(Math.round(rightBox.y))
+      // Equal width (tolerate sub-pixel differences)
+      expect(Math.round(leftBox.width)).toEqual(Math.round(rightBox.width))
+    }).toPass()
+  })
 })

test/fields/collections/Row/index.ts

@@ -137,6 +137,47 @@ const RowFields: CollectionConfig = {
         },
       ],
     },
+    {
+      type: 'row',
+      fields: [
+        {
+          name: 'leftColumn',
+          type: 'blocks',
+          blocks: [
+            {
+              slug: 'leftTextBlock',
+              fields: [
+                {
+                  name: 'leftText',
+                  type: 'text',
+                },
+              ],
+            },
+          ],
+          admin: {
+            width: '50%',
+          },
+        },
+        {
+          name: 'rightColumn',
+          type: 'blocks',
+          blocks: [
+            {
+              slug: 'rightTextBlock',
+              fields: [
+                {
+                  name: 'rightText',
+                  type: 'text',
+                },
+              ],
+            },
+          ],
+          admin: {
+            width: '50%',
+          },
+        },
+      ],
+    },
   ],
 }
 

test/fields/payload-types.ts

@@ -1129,6 +1129,22 @@ export interface RowField {
   no_set_width_within_row_b?: string | null;
   no_set_width_within_row_c?: string | null;
   field_20_percent_width_within_row_d?: string | null;
+  leftColumn?:
+    | {
+        leftText?: string | null;
+        id?: string | null;
+        blockName?: string | null;
+        blockType: 'leftTextBlock';
+      }[]
+    | null;
+  rightColumn?:
+    | {
+        rightText?: string | null;
+        id?: string | null;
+        blockName?: string | null;
+        blockType: 'rightTextBlock';
+      }[]
+    | null;
   updatedAt: string;
   createdAt: string;
 }
@@ -2760,6 +2776,28 @@ export interface RowFieldsSelect<T extends boolean = true> {
   no_set_width_within_row_b?: T;
   no_set_width_within_row_c?: T;
   field_20_percent_width_within_row_d?: T;
+  leftColumn?:
+    | T
+    | {
+        leftTextBlock?:
+          | T
+          | {
+              leftText?: T;
+              id?: T;
+              blockName?: T;
+            };
+      };
+  rightColumn?:
+    | T
+    | {
+        rightTextBlock?:
+          | T
+          | {
+              rightText?: T;
+              id?: T;
+              blockName?: T;
+            };
+      };
   updatedAt?: T;
   createdAt?: T;
 }