Input Interaction: fix buffer for the triggering of multi-select

packages/dom/src/dom.js

@@ -180,9 +180,15 @@ export function isVerticalEdge( container, isReverse ) {
 		return false;
 	}
 
-	const buffer = rangeRect.height / 2;
 	const editableRect = container.getBoundingClientRect();
 
+	// Calculate a buffer that is half the line height. In some browsers, the
+	// selection rectangle may not fill the entire height of the line, so we add
+	// half the line height to the selection rectangle to ensure that it is well
+	// over its line boundary.
+	const { lineHeight } = window.getComputedStyle( container );
+	const buffer = parseInt( lineHeight, 10 ) / 2;
+
 	// Too low.
 	if ( isReverse && rangeRect.top - buffer > editableRect.top ) {
 		return false;

packages/e2e-tests/specs/__snapshots__/multi-block-selection.test.js.snap

@@ -0,0 +1,11 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Multi-block selection should only trigger multi-selection when at the end 1`] = `
+"<!-- wp:paragraph -->
+<p>1.</p>
+<!-- /wp:paragraph -->
+
+<!-- wp:paragraph -->
+<p></p>
+<!-- /wp:paragraph -->"
+`;

packages/e2e-tests/specs/multi-block-selection.test.js

@@ -6,6 +6,8 @@ import {
 	insertBlock,
 	createNewPost,
 	pressKeyWithModifier,
+	pressKeyTimes,
+	getEditedPostContent,
 } from '@wordpress/e2e-test-utils';
 
 describe( 'Multi-block selection', () => {
@@ -137,4 +139,30 @@ describe( 'Multi-block selection', () => {
 		const speakTextContent = await page.$eval( '#a11y-speak-assertive', ( element ) => element.textContent );
 		expect( speakTextContent.trim() ).toEqual( '3 blocks selected.' );
 	} );
+
+	// See #14448: an incorrect buffer may trigger multi-selection too soon.
+	it( 'should only trigger multi-selection when at the end', async () => {
+		// Create a paragraph with four lines.
+		await clickBlockAppender();
+		await page.keyboard.type( '1.' );
+		await pressKeyWithModifier( 'shift', 'Enter' );
+		await page.keyboard.type( '2.' );
+		await pressKeyWithModifier( 'shift', 'Enter' );
+		await page.keyboard.type( '3.' );
+		await pressKeyWithModifier( 'shift', 'Enter' );
+		await page.keyboard.type( '4.' );
+		// Create a second block.
+		await page.keyboard.press( 'Enter' );
+		// Move to the middle of the first line.
+		await pressKeyTimes( 'ArrowUp', 4 );
+		await page.keyboard.press( 'ArrowRight' );
+		// Select mid line one to mid line four.
+		await pressKeyWithModifier( 'shift', 'ArrowDown' );
+		await pressKeyWithModifier( 'shift', 'ArrowDown' );
+		await pressKeyWithModifier( 'shift', 'ArrowDown' );
+		// Delete the text to see if the selection was correct.
+		await page.keyboard.press( 'Backspace' );
+
+		expect( await getEditedPostContent() ).toMatchSnapshot();
+	} );
 } );