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Fix incorrect ClientSize calculation in PerMonitorV2 mode when form moves between monitors with different DPI #14277

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Fix incorrect ClientSize calculation in PerMonitorV2 mode when form moves between monitors with different DPI #14277

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35 changes: 33 additions & 2 deletions 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 @@ -4589,8 +4589,39 @@ protected virtual bool OnGetDpiScaledSize(int deviceDpiOld, int deviceDpiNew, re
SizeF currentAutoScaleDimensions = GetCurrentAutoScaleDimensions(fontwrapper.Handle);
SizeF autoScaleFactor = GetCurrentAutoScaleFactor(currentAutoScaleDimensions, AutoScaleDimensions);

desiredSize.Width = (int)(Size.Width * autoScaleFactor.Width);
desiredSize.Height = (int)(Size.Height * autoScaleFactor.Height);
// We should not include the window adornments (non-client area) in the autoScaleFactor calculation,
// because Windows scales them linearly by DPI ratio. We need to:
// 1. Get the non-client area size at both old and new DPI
// 2. Subtract old non-client area from current Size to get client area
// 3. Scale client area by autoScaleFactor
// 4. Add new non-client area to get the final desired size
CreateParams cp = CreateParams;
RECT adornmentsAtOldDpi = default;
RECT adornmentsAtNewDpi = default;

WINDOW_STYLE style = (WINDOW_STYLE)cp.Style;
WINDOW_EX_STYLE exStyle = (WINDOW_EX_STYLE)cp.ExStyle;

if (OsVersion.IsWindows10_1703OrGreater())
{
PInvoke.AdjustWindowRectExForDpi(ref adornmentsAtOldDpi, style, false, exStyle, (uint)deviceDpiOld);
PInvoke.AdjustWindowRectExForDpi(ref adornmentsAtNewDpi, style, false, exStyle, (uint)deviceDpiNew);
}
else
{
PInvoke.AdjustWindowRectEx(ref adornmentsAtOldDpi, style, false, exStyle);
adornmentsAtNewDpi = adornmentsAtOldDpi;
}

// Calculate client area at old DPI
int clientWidth = Size.Width - adornmentsAtOldDpi.Width;
int clientHeight = Size.Height - adornmentsAtOldDpi.Height;

// Scale client area by autoScaleFactor and add new DPI's non-client area.
// Use Math.Round to minimize rounding errors during DPI transitions.
desiredSize.Width = (int)Math.Round(clientWidth * autoScaleFactor.Width) + adornmentsAtNewDpi.Width;
desiredSize.Height = (int)Math.Round(clientHeight * autoScaleFactor.Height) + adornmentsAtNewDpi.Height;

Debug.WriteLine($"AutoScaleFactor computed for new DPI = {autoScaleFactor.Width} - {autoScaleFactor.Height}");

// Notify Windows that the top-level window size should be based on AutoScaleMode value.
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