Add documentation for viewport DPI fix

Co-authored-by: philstopford <1983851+philstopford@users.noreply.github.com>

Author: Copilot

VIEWPORT_DPI_FIX.md

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+# Viewport High DPI Scaling Fix
+
+## Problem Statement
+On scaled high DPI displays, the viewport geometry was not fitted correctly and the view was not centered. Objects would appear smaller than expected, and zoom-to-extents would not properly fit the geometry to the viewport.
+
+## Root Cause
+The viewport code was inconsistently using physical pixel dimensions (`RenderWidth`/`RenderHeight`) and logical dimensions (`Width`/`Height`) throughout the rendering pipeline.
+
+On high DPI displays:
+- **Logical dimensions** (`Width`/`Height`): The control size in logical pixels (e.g., 800x600)
+- **Physical dimensions** (`RenderWidth`/`RenderHeight`): The actual framebuffer size in physical pixels, scaled by DPI (e.g., 1600x1200 on a 2x DPI display)
+
+The issue was that some parts of the code used physical dimensions while others used logical dimensions, causing:
+1. The orthographic projection to be calculated based on physical pixels, making the view frustum too large in world space
+2. The zoom calculations to divide world space extents by physical pixels, making objects appear smaller
+3. Mouse coordinate conversions to scale by DPI when they should use logical coordinates
+
+## Solution
+Changed the viewport to consistently use **logical dimensions** throughout:
+
+### Files Modified
+
+#### 1. `Eto/Eto.VeldridSurface/VeldridDriver_Draw.cs`
+- **Draw() method**: Changed orthographic projection to use `Surface.Width`/`Surface.Height` instead of `Surface.RenderWidth`/`Surface.RenderHeight`
+- **drawGrid() method**: Changed grid extent calculations to use logical dimensions
+- **drawAxes() method**: Changed axis extent calculations to use logical dimensions
+
+#### 2. `Eto/Eto.VeldridSurface/VeldridDriver_Public.cs`
+- **zoomExtents() method**: Changed zoom level calculations to use `Surface.Width`/`Surface.Height` instead of `Surface.RenderWidth`/`Surface.RenderHeight`
+
+#### 3. `Eto/Eto.VeldridSurface/VeldridDriver_Conversions.cs`
+- **ScreenToWorld() method**: Changed coordinate conversions to use logical dimensions
+- **WorldToScreen() method**: Changed coordinate conversions to use logical dimensions
+
+#### 4. `Eto/Eto.VeldridSurface/VeldridDriver_Handlers.cs`
+- **dragHandler() method**: Removed scaling of mouse coordinates by `LogicalPixelSize`
+- **selectByClick() method**: Removed scaling of mouse coordinates by `LogicalPixelSize`
+
+## Technical Details
+
+### Before (Incorrect)
+```csharp
+// Orthographic projection using physical pixels
+float left = ovpSettings.getCameraX() - (float)Surface!.RenderWidth / 2 * zoom;
+float right = ovpSettings.getCameraX() + (float)Surface!.RenderWidth / 2 * zoom;
+
+// Zoom calculation using physical pixels
+float zoomLevel_x = dX / Surface!.RenderWidth;
+
+// Mouse coordinates scaled by DPI
+PointF scaledLocation = e.Location * Surface!.ParentWindow.LogicalPixelSize;
+```
+
+### After (Correct)
+```csharp
+// Orthographic projection using logical dimensions
+float left = ovpSettings.getCameraX() - (float)Surface!.Width / 2 * zoom;
+float right = ovpSettings.getCameraX() + (float)Surface!.Width / 2 * zoom;
+
+// Zoom calculation using logical dimensions
+float zoomLevel_x = dX / Surface!.Width;
+
+// Mouse coordinates in logical pixels (no scaling needed)
+PointF scaledLocation = e.Location;
+```
+
+## Why This Works
+
+1. **Orthographic Projection**: By using logical dimensions, the view frustum size in world space remains consistent regardless of DPI scaling. The GPU automatically handles the mapping to the physical framebuffer.
+
+2. **Zoom Calculations**: Using logical dimensions ensures that the same world space extent results in the same zoom level, regardless of DPI.
+
+3. **Coordinate Conversions**: Mouse events report coordinates in logical pixels, and by using logical dimensions throughout, we maintain consistency between input coordinates and rendered coordinates.
+
+4. **Framebuffer**: The framebuffer size (`RenderWidth`/`RenderHeight`) is still correctly set by the platform handlers (GTK/WPF) to match the physical pixel count, ensuring sharp rendering on high DPI displays.
+
+## Testing
+- All 437 existing unit tests pass
+- Build completes without errors
+- Code is ready for manual testing on different DPI settings (1x, 1.5x, 2x, etc.)
+
+## Expected Behavior After Fix
+- Geometry is properly fitted to the viewport on zoom-to-extents
+- View is correctly centered on the geometry
+- Zoom levels are consistent across different DPI settings
+- Mouse panning and selection work correctly on high DPI displays
+- Grid and axes render at appropriate scales