Fix position calc

packages/rich-text/src/apply-format.js

@@ -68,20 +68,25 @@ export function applyFormat(
 			};
 		}
 	} else {
+		// Determine the highest position the new format can be inserted at.
+		let position = +Infinity;
+
 		for ( let index = startIndex; index < endIndex; index++ ) {
 			if ( newFormats[ index ] ) {
 				newFormats[ index ] = newFormats[ index ]
 					.filter( ( { type } ) => type !== format.type );
+
+				const length = newFormats[ index ].length;
+
+				if ( length < position ) {
+					position = length;
+				}
 			} else {
 				newFormats[ index ] = [];
+				position = 0;
 			}
 		}
 
-		const position = Math.min(
-			newFormats[ startIndex ].length,
-			newFormats[ endIndex - 1 ].length
-		);
-
 		for ( let index = startIndex; index < endIndex; index++ ) {
 			newFormats[ index ].splice( position, 0, format );
 		}

packages/rich-text/src/test/apply-format.js

@@ -112,6 +112,22 @@ describe( 'applyFormat', () => {
 		expect( getSparseArrayLength( result.formats ) ).toBe( 3 );
 	} );
 
+	it( 'should apply format around existing format with break', () => {
+		const record = {
+			formats: [ , [ em ], , [ em ] ],
+			text: 'test',
+		};
+		const expected = {
+			...record,
+			formats: [ , [ strong, em ], [ strong ], [ strong, em ] ],
+		};
+		const result = applyFormat( deepFreeze( record ), strong, 1, 4 );
+
+		expect( result ).toEqual( expected );
+		expect( result ).not.toBe( record );
+		expect( getSparseArrayLength( result.formats ) ).toBe( 3 );
+	} );
+
 	it( 'should apply format crossing existing format', () => {
 		const record = {
 			formats: [ , , , , [ em ], [ em ], [ em ], , , , , , , ],