Fix ItemHeight of ListBox is not scaled on high DPI in OwnerDrawFixed mode

[ricardobossan]

Fixes #6382

Proposed changes

An OwnerDrawFixed ListBox keeps its fixed ItemHeight (the reporter's sample sets 25) when the
control is autoscaled. At higher DPI the font grows but the rows stay their design-time height, so the
items look cramped. .NET Framework scaled the item height; .NET (Core) doesn't.

• Scale the OwnerDrawFixed item height by factor.Height in ListBox.ScaleControl; the same
  scaling pass that already scales padding and margin. The result is clamped to [1, 255], and a
  height-axis guard keeps a partial pass from applying the factor twice.
• ScaleControl is the entry point that both AutoScaleMode.Font/Dpi and PerMonitorV2 route
  through, so no per-DPI-message hook is needed.

Customer Impact

• Owner-draw fixed ListBox items are the correct height at higher DPI instead of the design-time
  height. Long-standing customer report; does not occur on .NET Framework.

Regression?

• Versus .NET Framework: yes (Framework scaled it, Core didn't). Not a recent .NET regression.

Risk

• Low. Runs only for OwnerDrawFixed during a real scaling pass, reuses the ItemHeight setter, and
  clamps so it can't throw. Normal/OwnerDrawVariable unaffected.

Screenshots

Sample is HighDpiMode.SystemAware (reads DPI at startup): change the scale, then relaunch to

Before

150%: rows stay at ItemHeight = 25 while the font scales up (cramped).

After

150%: item height scales (25 > ~38), keeping the 100% proportions.

Test methodology

• Unit tests

Accessibility testing

• No accessibility surface change; item height only affects layout.

Test environment(s)

• 11.0.100-preview.5.26302.115

Microsoft Reviewers: Open in CodeFlow

[KlausLoeffelmann]

Let's move this to .NET 12 together with the TreeView API we had planned.
We should approach this for the next version more holistically, and see, at what other spots we need to do similar things.

@bartonjs FYI.

[bartonjs]

From an API review perspective, no worries, approved is approved... generally things aren't "approved so long as you get them in this version" (OK, sometimes that happens, like adding an abstract member, since it would be "so long as it's still the first version, but this is a preview-breaking change, so you need to doc it")

[Copilot]

Pull request overview

Addresses #6382 by ensuring ListBox.ItemHeight scales appropriately during autoscaling when DrawMode.OwnerDrawFixed is used, matching expected high-DPI behavior and avoiding cramped rows.

Changes:

• Update ListBox.ScaleControl to scale OwnerDrawFixed ItemHeight by factor.Height, guarded against double-application and clamped to [1, 255].
• Use midpoint rounding away from zero to preserve visual proportions at common DPI scale factors.
• Add unit tests covering scaling behavior, native handle synchronization, rounding behavior, exclusion-guard behavior, and clamping.

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/Controls/ListBoxes/ListBox.cs	Scales OwnerDrawFixed ItemHeight during autoscaling with rounding + clamping and a height-axis guard.
src/test/unit/System.Windows.Forms/System/Windows/Forms/ListBoxTests.cs	Adds unit test coverage for the new ScaleControl behavior and edge cases.

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