Hi everyone, we are encountering a bug, we can replicate this in version 5 to latest
To replicate the issue, select a row on the grid using checkbox selection then try to scroll down using mouse wheel.


[Screenshot]

^7FA9F2EE72C7488392A87BF02038D9DCDCEE2E494B41A7AE76^pimgpsh_fullsize_distr.png




Code:
Ext.onReady(function () {
    Ext.define('ForumThread', {
        extend: 'Ext.data.Model',
        fields: [
            'title', 'forumtitle', 'forumid', 'username', {
            name: 'replycount',
            type: 'int'
        }, {
            name: 'lastpost',
            mapping: 'lastpost',
            type: 'date',
            dateFormat: 'timestamp'
        },
            'lastposter', ' ', 'threadid']
    });


    var store = Ext.create('Ext.data.Store', {
        id: 'store',
        model: 'ForumThread',
        // allow the grid to interact with the paging scroller by buffering
        buffered: true,
        leadingBufferZone: 0,
        pageSize: 50,
        proxy: {
            type: 'jsonp',
            url: 'https://www.sencha.com/forum/remote_topics/index.php',
            reader: {
                root: 'topics',
                totalProperty: 'totalCount'
            }
        },
        autoLoad: true
    });
    var grid = Ext.create('Ext.grid.Panel', {
        width: 700,
        height: 500,
        title: '',
        viewConfig: {
                preserveScrollOnRefresh: true
            },
        //invalidateScrollerOnRefresh: true,
        store: store,
        loadMask: true,
        tbar: [{
            text: 'store.load()',
            listeners : {
                click: function() {
                    store.load();
                }
            }
        }],
        // grid columns
        columns: [{
            id: 'last',
            text: "Last Post",
            dataIndex: 'lastpost',
            flex: 1,
            renderer: Ext.util.Format.dateRenderer('n/j/Y g:i A'),
            sortable: false,
            groupable: false
        },{
            id: 'title',
            text: "Title",
            dataIndex: 'title',
            flex: 2,
            sortable: false,
            groupable: false
        }],
        renderTo: Ext.getBody(),
        selModel: Ext.create('Ext.selection.CheckboxModel', {
            selType: 'checkboxmodel'
        })
    });
    
    grid.down()
});