Fix white border artifact on maximized Form (Windows 11)

[Copilot]

On Windows 11 (particularly at fractional DPI scaling), maximizing a Form with UseSystemDecorations = false produces empty space at the window edges where content is not drawn and the window background shows through. The content is in the correct location but is clipped, and the top/left edge is not fully drawn.

Root Causes

Two separate root causes were identified and fixed:

1. Form always painted a 1px border even when maximized — Form.DefaultStyle sets Border.Width = 1, creating a visible colored strip at the screen edges and an offset that prevents content from filling the window corners.

2. Floating-point precision loss in DoPaint and OnResize — ScaledClientSize was computed as (int)(ClientSize.Width * RenderScaling). Because ClientSize.Width = GetClientRect.right / RenderScaling (a float division), the round-trip pixels ÷ scale × scale loses 1 pixel — e.g. 2576 / 1.5 × 1.5 = 2575.999… → (int) = 2575. This made the DrawBorder rectangle and ClipRect 1 pixel too small, leaving a strip of form background at the edges and clipping the top/left content pixel.

Changes

• No border when maximized (Form.cs) — Added a per-instance maximized_style (ControlStyle inheriting dynamically from the instance Style via parent chain) with Border.Width = 0. CurrentStyle is overridden to return it when WindowState == Maximized, causing ControlAdapter.OnPaint to use form_x = form_y = 0 so controls fill from the very edge of the framebuffer.

• Disable resize handling when maximized (Form.cs) — HandleMouseDown returns false immediately when maximized (prevents accidental resize drags from window edges). HandleMouseMove delegates to base when maximized (suppresses resize cursors).

• Use actual framebuffer dimensions in DoPaint (WindowBase.cs) — Replaced ScaledClientSize with framebuffer.Size.Width/Height for both DrawBorder and ClipRect, eliminating the float round-trip precision loss. Also fixed the ClipRect formula from the accidentally-correct Width+1 to the semantically correct fb_display_rect.Right/Bottom.

• Use Math.Ceiling in OnResize (WindowBase.cs) — Changed adapter.SetBounds from (int)size.Width to (int)Math.Ceiling(size.Width) so controls always cover the full framebuffer even when the logical window size is fractional (e.g. 2576 px / 1.5 = 1717.33 logical px).

// maximized_style suppresses the border when the window is maximized
maximized_style = new ControlStyle(Style);
maximized_style.Border.Width = 0;

public override ControlStyle CurrentStyle =>
    WindowState == FormWindowState.Maximized ? maximized_style : base.CurrentStyle;

// DoPaint: use exact framebuffer dimensions to avoid float round-trip loss
var fb_width = framebuffer.Size.Width;
var fb_height = framebuffer.Size.Height;
e.Canvas.DrawBorder(new Rectangle(0, 0, fb_width, fb_height), CurrentStyle);
e.Canvas.ClipRect(new SKRect(fb_display_rect.Left, fb_display_rect.Top,
                              fb_display_rect.Right, fb_display_rect.Bottom));

// OnResize: ceiling ensures controls fill the framebuffer at fractional DPI
adapter.SetBounds(DisplayRectangle.Left, DisplayRectangle.Top,
                  (int)Math.Ceiling(size.Width), (int)Math.Ceiling(size.Height));

[jpobst]

@copilot While these are good changes, it does not fix the "border" around the maximized window. Some additional information that might help solve the issue:

• It is not actually a border, it is empty space on the left and right side where we are not drawing content, and the window background is showing through.
• The content isn't simply moved down and right X pixels, it is in the correct location, however it is being clipped and the top/left edge is not getting fully drawn.
• This only happens when Form.UseSystemDecorations = false. That is, when the system draws the window titlebar it works correctly. This blank space only appears when the system decorations are turned off and we are using the default custom FormTitleBar instead.

[Copilot]

@copilot While these are good changes, it does not fix the "border" around the maximized window. Some additional information that might help solve the issue:

• It is not actually a border, it is em...

Thanks for the detailed explanation — that pointed me to two additional root causes I was missing.

Root cause 1 — Border drawn even when maximized (addressed in first commit): Form.DefaultStyle always paints a 1px border, which creates a visible colored strip at the maximized window edges. I added a maximized_style with Border.Width = 0 and override CurrentStyle to return it when maximized. I also disabled the resize drag/cursor logic when maximized.

Root cause 2 — Float precision loss in DoPaint and OnResize (commit 9b3dba8): This is the actual source of the "empty space on left/right" and "content clipped" behavior. The original code computed ScaledClientSize as (int)(ClientSize.Width * RenderScaling). Because ClientSize.Width = GetClientRect.right / RenderScaling (a float division), the round-trip pixels ÷ scale × scale loses 1 pixel — e.g. 2576 / 1.5 × 1.5 = 2575.999… → (int) = 2575. This made the DrawBorder rect and ClipRect 1 pixel too small, leaving a strip of form background showing at the edges, and the ClipRect starting inside the first content pixel, causing the top/left clipping artefact.

Fixes in WindowBase.cs:

• DoPaint: use framebuffer.Size.Width/Height directly (the exact pixel count from the OS) for DrawBorder and ClipRect; also fixed the ClipRect formula from the accidentally-correct Width+1 to the semantically correct fb_display_rect.Right/Bottom
• OnResize: use Math.Ceiling(size.Width/Height) instead of truncating (int) so controls always cover the full framebuffer