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

[Sathish-087]

Fixes #10422

Proposed changes

• Ensure non-modal forms (Show(this)) respect StartPosition = CenterParent by invoking AdjustFormPosition() during OnLoad when CenterParent is specified.
• Correct centering behavior under DPI transitions by invoking AdjustFormPosition() during WmDpiChanged for hidden forms with CenterParent.
• Prevent incorrect centering caused by stale DPI-based size calculations when switching monitors.

Customer Impact

• Fixes inconsistent positioning behavior between modal (ShowDialog) and non-modal (Show) forms.
• Ensures forms consistently open centered relative to their parent across different DPI environments and multi-monitor setups.

Regression?

• No

Risk

• Low: Changes are limited to form positioning logic and gated by StartPosition == CenterParent.

Screenshots

Before

CenterParentIssueDemo.mp4

After

CenterParentFixDemo.mp4

Test methodology

• Created repro with two forms (Form1 and Form2) using both Show(this) and ShowDialog(this) across primary and secondary monitors.
• Verified centering behavior before and after fix in mixed DPI environments.
• Stepped through OnLoad, OnHandleCreated, and WmDpiChanged to validate correct invocation of AdjustFormPosition.

Accessibility testing

• NA

Test environment(s)

• SDK: 11.0.100-preview.3.26170.106
• OS: Windows 11
• Multi-monitor setup with mixed DPI (100% and 150%)

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR updates WinForms Form positioning logic to make StartPosition = CenterParent behave consistently for both modal (ShowDialog) and modeless (Show(owner)) forms, especially when autoscaling and DPI changes occur before the form becomes visible.

Changes:

• Invoke AdjustFormPosition() during OnLoad when StartPosition is CenterParent, so modeless owned forms get centered after autoscaling.
• Invoke AdjustFormPosition() during WM_DPICHANGED handling for hidden CenterParent forms to correct centering across DPI transitions.
• Add unit tests validating that both ShowDialog(owner) and Show(owner) with CenterParent are centered on the parent.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/System.Windows.Forms/System/Windows/Forms/Form.cs	Centers CenterParent forms during OnLoad and re-centers hidden CenterParent forms after DPI changes.
src/test/unit/System.Windows.Forms/System/Windows/Forms/FormTests.cs	Adds unit tests asserting centering behavior for modal and modeless owned forms using CenterParent.

---

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[LeafShi1]

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]

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?

[LeafShi1]

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.

[LeafShi1]

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.