Fix managed TitleBar affecting Control placement

[Copilot]

With managed decorations (default), the FormTitleBar occupies the top of the client area, so controls at (0, 0) render behind it. With UseSystemDecorations = true, the OS title bar is non-client area and controls are fully visible. The two modes should behave consistently.

Changes

• ControlAdapter.DisplayRectangle: Override to offset the layout rectangle by the visible managed title bar height. This shifts the origin for dock, anchor, and flow layout below the title bar — matching system decoration behavior.
• ControlAdapter.OnLayout: After base layout runs, manually position the FormTitleBar at (0, 0) spanning full width, then offset ALL other controls (including non-docked, non-anchored, absolute-positioned controls) by the title bar height. This ensures controls with Dock = None and absolute positioning are also placed below the managed title bar.

Remaining work

The Form constructor needs to set TitleBar.Dock = DockStyle.None after adding the implicit title bar. Without this, the dock layout engine will reposition the title bar into the offset DisplayRectangle, creating circular placement. The FormTitleBar constructor retains Dock = DockStyle.Top as the default so standalone usage (e.g. TitleBarPanel sample) is unaffected. Additionally, the DisplayRectangle override may need to be removed to avoid double-offsetting docked controls (since OnLayout now offsets all controls post-layout).

// Form.cs constructor — add after AddImplicitControl:
TitleBar = Controls.AddImplicitControl(new FormTitleBar());
TitleBar.Dock = DockStyle.None;  // exclude from normal layout flow

[jpobst]

@copilot This fix is not complete. It works for docked and anchored controls, however it does not work for controls that do not have dock or anchor because the layout engine does not use ControlAdapter.DisplayRectangle to lay them out. It appears to always use {0, 0} instead of the DisplayRectangle. (In fact, if there are no docked or anchored controls it does not even call ControlAdapter.DisplayRectangle).

For this test form:

public MainForm ()
{
    UseSystemDecorations = true;

    Controls.Add (new Button { Height = 45, Dock = DockStyle.Top, Text = "Dock - Top", TextAlign = ContentAlignment.MiddleCenter });
    Controls.Add (new Button { Top = 55, Text = "Dock - None", TextAlign = ContentAlignment.MiddleCenter });
}

It display correctly with UseSystemDecorations = true (doesn't use the managed TitleBar):

However it is incorrect with UseSystemDecorations = false:

[Copilot]

@copilot This fix is not complete. It works for docked and anchored controls, however it does not work for controls that do not have dock or anchor because the layout engine does not u...

Fixed in e59d8fb. The OnLayout override now runs after base.OnLayout() and offsets ALL non-titlebar controls (including Dock = None / absolute-positioned ones) by the title bar height. Previously only docked/anchored controls were handled via DisplayRectangle.

Note: The FormTitleBar still has Dock = DockStyle.Top as its default, and the Form constructor doesn't set TitleBar.Dock = DockStyle.None yet. This may cause the title bar itself to be double-offset during layout. If that's the case, adding TitleBar.Dock = DockStyle.None in the Form constructor (after AddImplicitControl) would be needed to fully complete the fix.