Proper Resizing of SplitterContainer Controls at any DPI

Introduction

When a SplitterContainer control with a fixed panel is resized due to the DPI setting, the SplitterDistance value is not changed. This may result in the fixed panel appearing either too small or too large, depending on whether the DPI setting was increased or decreased.

For example, a Form with AutoScaleMode set to ScaleMode.DPI, and a SplitterContainer with FixedPanel set to Panel1 and SplitterDistance set to 140 appears as follows at it's native DPI (100%):

This is how it appears when run at a higher DPI (125%):

All the controls were resized, but SplitterDistance was left unchanged at 140.

Note that this is only a problem when SplitterContainer.FixedPanel is set to Panel1 or Panel2. If it was set to None, then SplitterDistance would be adjusted to match the resizing.

The solution to adjust SplitterDistance during the Form.Shown event handler. The resizing does not work properly if performed in the constructor or the Form.Load event handler.

Using the code

Include the following code snippet in your Form-derived class:

// Save the current scale value
// ScaleControl() is called during the Form's constructor
private SizeF scale = new SizeF(1.0f, 1.0f);
protected override void ScaleControl(SizeF factor, BoundsSpecified specified)
{
    scale = new SizeF(scale.Width * factor.Width, scale.Height * factor.Height);
    base.ScaleControl(factor, specified);
}

// Recursively search for SplitContainer controls
private void Fix(Control c)
{
    foreach (Control child in c.Controls)
    {
        if (child is SplitContainer)
        {
            SplitContainer sp = (SplitContainer)child;
            Fix(sp);
            Fix(sp.Panel1);
            Fix(sp.Panel2);
        }
        else
        {
            Fix(child);
        }
    }
}

private void Fix(SplitContainer sp)
{
    // Scale factor depends on orientation
    float sc = (sp.Orientation == Orientation.Vertical) ? scale.Width : scale.Height;
    if (sp.FixedPanel == FixedPanel.Panel1)
    {
        sp.SplitterDistance = (int)Math.Round((float)sp.SplitterDistance * sc);
    }
    else if (sp.FixedPanel == FixedPanel.Panel2)
    {
        int cs = (sp.Orientation == Orientation.Vertical) ? sp.Panel2.ClientSize.Width :sp.Panel2.ClientSize.Height;
        int newcs = (int)((float)cs * sc);
        sp.SplitterDistance -= (newcs - cs);
    }
}

Remember to call Fix(this) from the Form's Shown event handler. For Example:

private void Form1_Shown(object sender, EventArgs e)
{
    Fix(this);
} 

Points of Interest

I am puzzled as to why the SplitterDistance values must be adjusted during the <code>Form.Shown event handler, and not during Form.Load or the constructor. If anyone can explain this, please leave a comment.

History

• June 14, 2014 - First release.