Dual monitor dialog resize issue

Note: Considering my last bug report for the .NET Framework at the developer community has been migrated to the corresponding .NET Core project I report this issue directly here. Please note though that .NET Framework is also affected.

Issue reproduction steps:

1. Make sure your secondary display is left or above of your primary display (so it has negative relative coordinates)
2. Create a new Windows Forms Application and display a form by ShowDialog so the resize grip is visible.
3. Execute the application and drag the form to the secondary display.
4. Now as its coordinates are in the negative range, move the mouse along the border and see that the diagonal resize cursor appears all around the form. The issue is not just the displayed cursor but also the resizing behavior behavior: even clicking the top border the bottom-right corner will be resized

See also the screenshot below (the cursor is indicated by me).

Proposed fix:

The root cause lies in the Form.WmNCHitTest method. m.LParam contains two short values but the way of converting them to integers is wrong.

Original code:

// This is wrong for negative values. For example, -500 will be interpreted as 65036
int x = NativeMethods.Util.LOWORD(m.LParam);
int y = NativeMethods.Util.HIWORD(m.LParam);

Fixed code:

// LParam contains two shorts. Without the cast negative values turn positive ints
int x = (short)(m.LParam.ToInt32() & 0xFFFF);
int y = (short)((m.LParam.ToInt32() >> 16) & 0xFFFF);

[weltkante]

@koszeggy the daily build page doesn’t mention it but if you click the link the actual download section has 5.0 daily builds

[koszeggy]

I added the StatusBar fix to the PR. I didn’t change the ActiveX-related code found by @0xd4d though. Its test is a bit more complex and I think that would require another version of the MAKELPARAM method as well.