Why do multiple font sizes display inside Plain TextBox?

Question

This is not a question about how to change the font size. Rather, why is the size of my font changing by itself as I type (or paste) when it's inside a plain TextBox control which, as you should know, only supports one color, one font, and one font size at any given time.

My code:

using(FontDialog d = new FontDialog())
{
   // The usual properties...
   if(d.ShowDialog() == DialogResult.OK)
   {
      textbox1.Font = d.Font;
   }
}

The font is set at 8pt. If I paste plain text into it, the top line will be 9 to 10pt while the bottom line is noticeably smaller (about 8 pt).

It doesn't matter which font, font style, or font size I choose; this keeps happening!

Update

Thanks for all your help thus far.To answer your recent questions below:

My app is targetting .NET 4.5.

There's no mix up in the code, since I was able to reproduce this problem in a new Windows Forms project with nothing but a Form, a TextBox and a Button that calls the FontDialog.

To answer the question about my Video drivers, I did require support for an app I purchased a few weeks ago and they told me to run DXDiag, they got back to me saying my Video Card driver is out of date, however I didn't think it was because I always check every few months. I then went to the manufacturer's website and it said that I already have the latest drivers installed for my system.

Windows Update also says there are no new available updates. I'll check for a new version of drivers again, though.

---

I also did a test in a new blank project where I display the font being used by the TextBox before calling FontDialog.ShowDialog(), and after it has been shown and after the new font has been set and everything matches - yet there is still the issues after changing font/font size inside the textbox.

Answer 1

I suspect that the High-DPI screen that you are using is not using an exact multiple of 96 DPI. When in this situation Windows renders your application to a virtual screen that is 96 DPI and then rescales the result to the new DPI. Having a DPI that is not an exact multiple of 96 can cause rounding errors when the application's display is rescaled and as a result fonts can end up looking uneven.

Try setting your DPI to one of the sizes in Appendix C of Microsoft's Writing High-DPI Win32 Applications guide.