DataGridView rightmost column user increase size not working

Question

I have a strange issue with a DataGridView on a WinForm. I can resize all columns via mouse, but for the rightmost I can only shrink and not increase the size.

Answer 1

This is by design. When you Click/Hold your mouse button, the mouse is captured and cannot leave the client area, with the effect of blocking the resize. You have to give it a push.

I tried not to P/Invoke to release the capture.
See if this works for you (of course if any AutoSize mode is set, nothing will happen).

private bool IsLastColumn = false;
private bool IsMouseDown = false;
private int MouseLocationX = 0;

private void dataGridView1_MouseDown(object sender, MouseEventArgs e)
{
   var dgv = sender as DataGridView;
   int lastColumnIndex = dgv.Columns.Count - 1;

   //Check if the mouse pointer is contained in last column boundaries
   //In the middle of it because clicking the divider of the row before
   //the last may be seen as inside the last too.
   Point location = new Point(e.X - (dgv.Columns[lastColumnIndex].Width / 2), e.Y);
   if (dgv.GetColumnDisplayRectangle(lastColumnIndex, true).Contains(location))
   {
      //Store a positive checks and the current mouse position
      this.IsLastColumn = true;
      this.IsMouseDown = true;
      this.MouseLocationX = e.Location.X;
   }
}

private void dataGridView1_MouseMove(object sender, MouseEventArgs e)
{
   var dgv = sender as DataGridView;

   //If it's the last column and the left mouse button is pressed...
   if ((this.IsLastColumn) && (this.IsMouseDown) && (e.Button == MouseButtons.Left))
   {
      // Adding the width of the vertical scrollbar, if any.
      int cursorXPosition = e.X;
      if (dgv.Controls.OfType().Where(s => s.Visible).Count() > 0)
      {
          cursorXPosition += SystemInformation.VerticalScrollBarWidth;
      }

      //... calculate the offset of the movement.
      //You'll have to play with it a bit, though
      int colWidth = dgv.Columns[dgv.Columns.Count - 1].Width;
      int colWidthOffset = columnWidth + (e.X - this.MouseLocationX) > 0 ? (e.X - this.MouseLocationX) : 1;
      //If mouse pointer reaches the limit of the clientarea...
      if ((colWidthOffset > -1) && (cursorXPosition >= dgv.ClientSize.Width - 1))
      {
         //...resize the column and move the scrollbar offset
         dgv.HorizontalScrollingOffset = dgv.ClientSize.Width + colWidth + colWidthOffset;
         dgv.Columns[dgv.Columns.Count - 1].Width = colWidth + colWidthOffset;
      }
   }
}

private void dataGridView1_MouseUp(object sender, MouseEventArgs e)
{
    this.IsMouseDown = false;
    this.IsLastColumn = false;
}