Drag Preview Content and Opacity

[wieslawsoltes]

Drag Preview Content and Opacity

Summary

This change adds full drag preview support for IDock layouts, including the ability to render dockable content during drag and to control preview opacity via DockSettings. The preview window now renders dock containers using the same default data templates that DockControl injects, preventing view locator failures and enabling nested layouts to display correctly while dragging.

Implementation details

• Added new settings:
  • DockSettings.ShowDockablePreviewOnDrag to toggle rendering dockable content inside the preview window.
  • DockSettings.DragPreviewOpacity to control preview window opacity.
• Extended DragPreviewControl with:
  • ShowContent, PreviewContentWidth, and PreviewContentHeight properties for sizing and toggling the content preview.
• Updated DragPreviewHelper to:
  • Initialize default dock DataTemplates for DragPreviewControl using DockDataTemplateHelper.
  • Compute preview sizing from tracked dockable bounds with fallback defaults.
  • Apply clamped opacity (0.0 to 1.0) to the preview window.
• Updated the Fluent theme preview template to display both status and optional dockable content.
• Wired the ReactiveUI sample to enable dockable previews and set preview opacity.

Usage

Enable dockable preview content:

DockSettings.ShowDockablePreviewOnDrag = true;

Set preview opacity (0.0 to 1.0):

DockSettings.DragPreviewOpacity = 0.6;

AppBuilder shortcuts:

AppBuilder.Configure<App>()
    .UsePlatformDetect()
    .ShowDockablePreviewOnDrag()
    .SetDragPreviewOpacity(0.6);

Tests

• dotnet test

Fixes #339