WinForms layout issue with CheckBox and ComboBox when using HighDpiMode.PerMonitorV2

[Aniobodo]

.NET version

.NET 10

Did it work in .NET Framework?

Not tested/verified

Did it work in any of the earlier releases of .NET Core or .NET 5+?

No response

Issue description

When a UserControl contains a CheckBox and a ComboBox, enabling
Application.SetHighDpiMode(HighDpiMode.PerMonitorV2);
causes a height mismatch between the controls. The ComboBox appears slightly taller, which results in two visible lines at the bottom of the control.

This only occurs when PerMonitorV2 DPI awareness is enabled.

Steps to reproduce

(A video and a VS project are attached below as demo)
Create a UserControl containing: one CheckBox and one ComboBox

Example startup code:

[STAThread]
static void Main()
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Application.SetHighDpiMode(HighDpiMode.PerMonitorV2);

    Application.Run(new Form1());
}

Place both controls on a UserControl, aligned vertically.

https://github.com/user-attachments/assets/4e98643b-8c81-462f-844a-80f9233079b1
CAD_VS2026.zip

[Olina-Zhang]

Hi @Aniobodo Tested the app you provided, found the problem root cause is AutoScaleDimensions mismatch caused DPI scaling conflicts when DPI is greater than 100%DPI:
• MyUserControl: (8F, 23F) → larger font baseline
• CADDlg Form: (7F, 15F) → smaller font baseline
In high DPI, both controls scaled independently, causing ComboBox to be taller than UserControl container.
Additional issue: ComboBoxStyle.Simple requires different height calculation (EditBox + ListBox area), but original code used PreferredHeight which only returns EditBox height.

Did some changes:

1. MyUserControl.Designer.cs

// Before:
AutoScaleDimensions = new SizeF(8F, 23F);
Size = new Size(254, 31);
SizeChanged += SuperCombo_SizeChanged;

// After:
AutoScaleDimensions = new SizeF(7F, 15F);  // ✅ Unified with parent Form
Size = new Size(254, 23);                  // ✅ Matches ComboBox height
// ✅ Removed event handler binding

2. MyUserControl.cs:
   Removed:
   • SuperCombo_SizeChanged() event handler (unstable)
   • SuperCombo_SizeChanged_EventSuspended flag
   Added:
   • GetPreferredControlHeight() - Smart height calculation:
   Simple style: PreferredHeight + DropDownHeight (~223px)
   Other styles: PreferredHeight only (~23px)

• SetBoundsCore() override - DPI scaling fix:
Non-Simple styles: Force height = PreferredHeight
Simple style: Respect designer height (skip override)

• OnResize() override:
Simple style: Set ComboBox.Height = this.Height (fill container)
Other styles: Width auto-fill only
• AdjustControlHeight() - Triggered when DropDownStyle changes

In order to:
✅ Unified DPI baseline - No scaling conflicts
✅ DropDown/DropDownList - Perfect height match at any DPI
✅ Simple style - Full height including list area
✅ Reliable - Uses SetBoundsCore() instead of event handlers

Modified app:
CAD_VS2026_modified.zip, can you try it?

[Aniobodo]

@Olina-Zhang Thank you for the checks.
With exactly the CAD_VS2026_modified.zip, it works fine. But setting the main form's AutoScaleDimensions = new SizeF(7F, 15F);, the problem returns and persists even with smaller UserControl size .Please see the video here https://drive.google.com/open?id=16S3k29G8MijmVze8RSHV1Pao9NrHjHWF&usp=drive_fs

[Olina-Zhang]

@Aniobodo In Windows Forms, the AutoScaleDimensions property stores the design-time scaling baseline (such as DPI or font size) used by the form when it was created in the designer.
At runtime, WinForms compares AutoScaleDimensions with CurrentAutoScaleDimensions to determine whether controls need to be scaled.
AutoScaleDimensions should not be modified manually, as this value is automatically generated by the designer, and changing it manually may cause incorrect scaling behavior.
In your scenario, if the application is running on a system with a DPI setting different from the design-time DPI and the AutoScaleDimensions value was modified incorrectly, the recommended approach is to reopen the form designer in the Visual Studio, make a small change to the form (for example resizing it), and allow the designer to regenerate the correct AutoScaleDimensions value.
In general, the recommended practice is to design WinForms UI at 100% DPI (96 DPI) and then test the application at different DPI settings to ensure the layout scales correctly.

[Aniobodo]

@Olina-Zhang

AutoScaleDimensions should not be modified manually

: That was a test because a form where the UserControl is placed already exist with AutoScaleDimensions = new SizeF(7F, 15F).

---

My initial report that https://github.com/user-attachments/files/25889440/CAD_VS2026_modified.zip works was from a test on DeviceDpi = 192. Now, without any modification, I opened the project on a DeviceDpi = 96, and the result is bad. The bad result persists even with one small size adjustment (aimed at informing the designer to detect the change of device). Here is the video about that: https://drive.google.com/open?id=1mj6892sSMQI-mg6gSKU5fYjpY3VxSzpr&usp=drive_fs

[Olina-Zhang]

@Aniobodo that modified app based on my machine: 200%DPI. If the DPI is not same, looks like loading form designer and making a small change just ensure the comboBox with DropDownStyle: Dropdown work well, the ComboBox with DropDownStyle: Simple still has layout issue. The simple fix is re-adding userControl from ToolBox to form designer again.

[Aniobodo]

@Olina-Zhang Re-adding userControl from ToolBox to form designer again is not a simple solution when you have numerous controls. Moreover, it still does not solve the underlying problem with Application.SetHighDpiMode(HighDpiMode.PerMonitorV2);

[Olina-Zhang]

Your issue exists in both PerMonitorV2 and SystemAware modes. This requires further investigation by our development team to determine whether it is a project-level or product-level issue.

[LeafShi1]

If the intent is to keep the UserControl height aligned with the single-line height of the inner ComboBox, layout adjustments should be performed in OnLayout rather than by repeatedly modifying Size inside a SizeChanged event handler.
Mutating Size during SizeChanged can easily conflict with WinForms layout and autoscaling, particularly in high-DPI or PerMonitorV2 scenarios.
In addition, please ensure that the UserControl and its hosting Form use consistent design-time AutoScaleDimensions to avoid scaling baseline mismatches.

protected override void OnLayout(LayoutEventArgs e)
{
    base.OnLayout(e);

    //  Simple: do not adjust height (the list area is always visible)
    if (ComboBoxx.DropDownStyle == ComboBoxStyle.Simple)
        return;

    // Non-Simple: lock the height to the single-line ComboBox height
    int desiredHeight = ComboBoxx.PreferredHeight;

    if (Height != desiredHeight)
    {
        // Adjust only when necessary to avoid recursion
        Height = desiredHeight;
    }
}

[Aniobodo]

@LeafShi1
With OnLayout above added, SizeChanged event removed and at AutoScaleDimensions = new System.Drawing.SizeF(7F, 15F) for both UserControl and its hosting Form: the problem remains, see image below.