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Fix HDPI: Position of a Modal-Form (and Modeless) is not centered to parent #14664

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Fix HDPI: Position of a Modal-Form (and Modeless) is not centered to parent #14664

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fb3b91e
Added fix for issue 10422
Sathish-087 Jun 11, 2026
26fc742
Merge branch 'dotnet:main' into Fix_Issue_10422
Sathish-087 Jun 22, 2026
236d1a1
Address review comments and Added unit test cases
Sathish-087 Jun 22, 2026
dee1450
Remove unwanted comments
Sathish-087 Jun 22, 2026
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6 changes: 5 additions & 1 deletion src/System.Windows.Forms/System/Windows/Forms/Form.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -4290,7 +4290,7 @@ protected virtual void OnLoad(EventArgs e)

// Also, at this time we can now locate the form on the correct area of the screen.
// We must do this after applying any autoscaling.
if (GetState(States.Modal))
if (GetState(States.Modal) || (FormStartPosition)_formState[s_formStateStartPos] == FormStartPosition.CenterParent)

@LeafShi1 LeafShi1 Jun 24, 2026 •
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In a cross-DPI scenario (e.g., parent is on a hi-DPI monitor), the sequence could be:

  1. CreateHandle() — window created on primary monitor
  2. WM_DPICHANGED fires (!Visible && CenterParent) → AdjustFormPosition()
  3. OnLoad fires (CenterParent) → AdjustFormPosition() again

This results in AdjustFormPosition() being called twice. It’s harmless (idempotent), but worth noting. Consider adding a comment here explaining why the double call is acceptable, or adding a guard to skip if already adjusted.

@LeafShi1 LeafShi1 Jun 24, 2026 •
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Previously only modal forms (ShowDialog) reached this path, and ShowDialog always has an owner. Now non-modal Show() can also reach here. What happens if the user calls:

childForm.StartPosition = FormStartPosition.CenterParent;
childForm.Show();  // no owner

Does AdjustFormPosition() handle Owner == null gracefully? Could you verify this edge case doesn’t throw or produce unexpected positioning?

{
AdjustFormPosition();
}
Expand Down Expand Up @@ -4564,6 +4564,10 @@ private void WmDpiChanged(ref Message m)
DeviceDpiInternal = e.DeviceDpiNew;

OnDpiChanged(e);
if (!Visible && (FormStartPosition)_formState[s_formStateStartPos] == FormStartPosition.CenterParent)
{
AdjustFormPosition();

@LeafShi1 LeafShi1 Jun 24, 2026 •
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The !Visible guard is correct — it prevents overriding the user’s position after the form is shown. However, the intent isn’t immediately obvious to future readers. Consider adding a comment, e.g.:

// Only re-center before the form is shown to the user. Once visible,
// the user may have repositioned it and we must not override that.

}
}

/// <summary>
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71 changes: 71 additions & 0 deletions src/test/unit/System.Windows.Forms/System/Windows/Forms/FormTests.cs
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Expand Up @@ -2763,6 +2763,77 @@ public void Form_DoesNot_DisposeUserIcon()
smallIcon.Handle.Should().NotBe(0);
}

[WinFormsFact]
public void Form_ShowDialog_WithCenterParent_PositionsCenteredOnParent()
{
using Form parentForm = new()
{
Size = new Size(400, 400),
StartPosition = FormStartPosition.Manual,
Location = new Point(200, 200)
};

parentForm.Show();
using Form childForm = new()
{
Size = new Size(200, 200),
StartPosition = FormStartPosition.CenterParent
};

Point capturedLocation = Point.Empty;
childForm.Load += (sender, e) =>
{
capturedLocation = childForm.Location;
childForm.Close();
};

childForm.ShowDialog(parentForm);
// Expected center: parentForm center minus half of childForm size.

@LeafShi1 LeafShi1 Jun 24, 2026 •
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childForm.Width and childForm.Height are read after ShowDialog returns (i.e., after Close() was called inside the Load handler). If Close() triggers a WM_DPICHANGED (e.g., the form moves to a different monitor during teardown), the size could change.

Consider capturing the size inside the Load handler alongside the location:

Size capturedSize = Size.Empty;
childForm.Load += (sender, e) =>
{
    capturedLocation = childForm.Location;
    capturedSize = childForm.Size;
    childForm.Close();
};

Then use capturedSize for the expected position calculation.

int expectedX = parentForm.Left + (parentForm.Width - childForm.Width) / 2;
int expectedY = parentForm.Top + (parentForm.Height - childForm.Height) / 2;
int deltaX = SystemInformation.Border3DSize.Width + 1;
int deltaY = SystemInformation.Border3DSize.Height + 1;

capturedLocation.X.Should().BeInRange(expectedX - deltaX, expectedX + deltaX);
capturedLocation.Y.Should().BeInRange(expectedY - deltaY, expectedY + deltaY);
}

[WinFormsFact]
public void Form_Show_WithCenterParent_PositionsCenteredOnParent()
{
using Form parentForm = new()
{
Size = new Size(400, 400),
StartPosition = FormStartPosition.Manual,
Location = new Point(200, 200)
};

parentForm.Show();
using Form childForm = new()
{
Size = new Size(200, 200),
StartPosition = FormStartPosition.CenterParent
};

Point capturedLocation = Point.Empty;
childForm.Load += (sender, e) =>
{
capturedLocation = childForm.Location;
};

childForm.Show(parentForm);
childForm.Close();

// Expected center: parentForm center minus half of childForm size.
int expectedX = parentForm.Left + (parentForm.Width - childForm.Width) / 2;
int expectedY = parentForm.Top + (parentForm.Height - childForm.Height) / 2;
int deltaX = SystemInformation.Border3DSize.Width + 1;
int deltaY = SystemInformation.Border3DSize.Height + 1;

capturedLocation.X.Should().BeInRange(expectedX - deltaX, expectedX + deltaX);
capturedLocation.Y.Should().BeInRange(expectedY - deltaY, expectedY + deltaY);
}

public partial class ParentedForm : Form
{
private ParentingForm _parentForm;
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