src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/contains
class EditorGroupModel:	contains(candidate: EditorInput | IUntypedEditorInput, options?: { supportSideBySide?: boolean, strictEquals?: boolean }): boolean {
		for (const editor of this.editors) {
			if (this.matches(editor, candidate, options?.strictEquals)) {
				return true;
			}

			if (options?.supportSideBySide && editor instanceof SideBySideEditorInput) {
				if (this.matches(editor.primary, candidate, options?.strictEquals) || this.matches(editor.secondary, candidate, options?.strictEquals)) {
					return true;
				}
			}
		}

		return false;
	}