Corruption of Table.topIndex when deleting items off the bottom of the list, and using BeginUpdate().

[antiduh]

I've found a somewhat complicated bug involving Table.TopIndex - under certain usages, it seems that the value gets corrupted, causing the table to layout rows in the wrong order.

Specifically, the circumstances to reproduce it are as follows:

• Add enough rows to a table such that there are rows below the bottom visible edge of the table.
• Scroll down and remove the last row.
• Note that sometimes the table does not scroll up to remove empty whitespace at the bottom of the table created by removing the last row.
• Repeat removing the last row until rows are deleted offscreen.
• Table.topIndex is now corrupted, and the scrollbars/row layout act very strangely.

It seems that to reproduce this it is necessary to use BeginUpdate() and EndUpdate(). Specifically, whenever my "data model" changes, I "update" the table by clearing it entirely and refilling it:

private void ReloadTable()
{
    int selectedRow = SelectedRowIndex;

    table.BeginUpdate();

    tableModel.Rows.Clear();

    foreach( int value in data )
    {
        Row row = new Row();
        row.Cells.Add( new Cell( value ) );
        tableModel.Rows.Add( row );
    }

    table.EndUpdate();

    // Restore selection.
    if( selectedRow != -1 )
    {
        int row = Math.Min( selectedRow, tableModel.Rows.Count - 1 );

        tableModel.Selections.Clear();
        tableModel.Selections.SelectCell( row, 0 );
    }
}

Since this is a slightly complicated bug to understand, I've attached a demo program and a video of me reproducing the bug. In order to make the demo program work, I've had to make a few mods to Table.cs for debugging (be able to modify TopIndex and also catch an event when it changes).

I've attached a zip file containing my demo program and a video of me reproducing the bug, as well as a patch file showing the changes I needed to make to Table.cs to make the demo program.

XPTable TopIndex.zip

[antiduh]

I was able to mitigate this by changing the implementation of vScrollBar_ValueChanged:

private void vScrollBar_ValueChanged( object sender, EventArgs e )
{
    // Work around for https://github.com/schoetbi/XPTable/issues/39
    // Instead of the complicated GetNewTopRowIndex/FindNextVisibleCell bs, 
    // just use the scrollbar value; that's what its there for.
    //int newtopIndex = GetNewTopRowIndex(topIndex, vScrollBar.Value - lastVScrollValue);
    //topIndex = newtopIndex;

    this.topIndex = vScrollBar.Value;

    this.Invalidate();

    if( this.EnableWordWrap )
        UpdateScrollBars();

    lastVScrollValue = vScrollBar.Value;
}

I'm not sure why my implementation isn't what is used to start with - my understanding is that the scrollbar's value and topIndex should always be in sync. Perhaps it has to do with hidden rows or parent-child rows?

It works for me, but I'm not sure the above fix can be committed.

[schoetbi]

I also have an odd feeling when changing scrollbar issues since the code is not easy understandable and clear. Please give me some time to look at it.