Fix focusing block widgets inside multi root editor

[Mati365]

Suggested merge commit message (convention)

Fix (editor-multi-root): No longer lose selection on clicking editable containing only one block element. Closes #16806

---

Additional information

I adjusted multi-root demos with additional plugins to make debugging block items easier.
Debug log: #16806 (comment)

Before

chrome-selection-before-2024-08-09_09.00.01.mp4

After

chrome-selection-fix-2024-08-09_08.57.31.mp4

chrome-selection-images-2024-08-09_11.02.41.mp4

[niegowski]

Widgets have a dedicated handling of the selection. The DOM selection is handled differently while a widget is selected, it's anchored in a fake selection container. I feel like the fix should be located in this handler of the widget:

ckeditor5/packages/ckeditor5-widget/src/widget.ts

Lines 276 to 320 in dcb087f

	private _onMousedown( eventInfo: EventInfo, domEventData: DomEventData<MouseEvent> ) {
	const editor = this.editor;
	const view = editor.editing.view;
	const viewDocument = view.document;
	let element: ViewElement | null = domEventData.target;
	// If triple click should select entire paragraph.
	if ( domEventData.domEvent.detail >= 3 ) {
	if ( this._selectBlockContent( element ) ) {
	domEventData.preventDefault();
	}
	return;
	}
	// Do nothing for single or double click inside nested editable.
	if ( isInsideNestedEditable( element ) ) {
	return;
	}
	// If target is not a widget element - check if one of the ancestors is.
	if ( !isWidget( element ) ) {
	element = element.findAncestor( isWidget );
	if ( !element ) {
	return;
	}
	}
	// On Android selection would jump to the first table cell, on other devices
	// we can't block it (and don't need to) because of drag and drop support.
	if ( env.isAndroid ) {
	domEventData.preventDefault();
	}
	// Focus editor if is not focused already.
	if ( !viewDocument.isFocused ) {
	view.focus();
	}
	// Create model selection over widget.
	const modelElement = editor.editing.mapper.toModelElement( element );
	this._setSelectionOverElement( modelElement! );
	}

[Mati365]

Unfortunately it's still a bit buggy :/ It looks like, while cursor is properly placed in element, the table is receiving incorrectly focus. Probably related to element.childCount check.

bug.mp4

[Mati365]

It looks like incorrect istanbul report, because return false; is reached during the tests.

[niegowski]

Partial review.

[Mati365]

@niegowski I moved function to utils. Can you take a look?

[niegowski]

Please explain what the original comment was about.

[kubaklatt]

It is not a regression, but I managed to find two more scenarios that cause identical problem on Safari - #16978

[charlttsie]

The issue still reproduces on Chrome when using tab to switch to a root with a block widget only - #16981