fix(dragdrop): Load the drag visual - #25870

GitHub Issue: closes #25869

PR Type:

🐞 Bugfix

What changed? 🚀

The drag layer (DragRoot) now enters the live tree when InputManager.OpenDragAndDrop adds it, and leaves it before it's removed. The drag visual is loaded while it's shown, so it shows the dragged element again.

Before.

• OpenDragAndDrop added the drag layer as a child of the visual tree's root element, which isn't active in the visual tree itself.
• VisualTree.AddRoot enters each of its other roots explicitly (root.Enter(new EnterParams(isLive: true), 0)), but the drag layer was added without that. The drag visual was never live, and never loaded.
• Since fix(imaging): Release a decoded image when nothing displays it #25124, an Image only subscribes to its source once it's loaded, as WinUI only decodes in the live tree. So the drag visual's Image showed nothing:
  • not the snapshot of the dragged element that Uno renders when the app sets no content in DragStarting, so no in-app drag showed the dragged element, only the caption tooltip;
  • not the content an app set with DragUI.SetContentFromBitmapImage or DragUIOverride.SetContentFromBitmapImage.

After.

• OpenDragAndDrop enters the drag layer, live, right after adding it, as VisualTree.AddRoot does for the other roots. It skips that if the layer is already active, in case the root element is active.
• When the last drag ends, it leaves the drag layer, live, before removing it, as WinUI's VisualTree::RemoveRoot does. The layer doesn't stay marked active after it's removed.
• The drag layer and the drag visual get Loaded, then Unloaded when the drag ends, and the drag visual's image shows. The drag layer stays out of hit-testing (IsHitTestVisible is false).
• Being loaded now also applies the drag visual's theme resources and visual states. I compared it in the light and dark application themes, before and after: the caption tooltip looks the same in each theme, and the image now shows.

Platforms: all, since the change is in shared code. Verified on Skia Desktop (Windows).

Tests: Given_DragView.When_Shown_Then_Loaded_And_Content_Image_Decodes shows a drag visual twice in a row through InputManager.OpenDragAndDrop, as the drag layer is removed after the first and added again for the second. Each time it has a BitmapImage (ms-appx:///Assets/square100.png) that nothing else shows. Each time, the test checks that:

• the drag visual is loaded;
• the image decodes (PixelWidth > 0);
• the drag layer isn't hit-testable;
• once the drag visual is closed, the drag visual is unloaded and the drag layer is no longer active.

On Skia Desktop (Windows), on master at 1f2fc30:

I also ran Given_DragView, Given_Win32DragDrop, and the drag-and-drop tests of Given_UIElement, Given_ListViewBase and Given_TreeView: 25 tests, the new one included, all passing, after a build with the analyzers on.

Backport: not needed in servicing/6.8 unless #25124's Image change is backported there.

PR Checklist ✅

• 🧪 Added Runtime tests, UI tests, or a manual test sample (for bug fixes / features, if applicable)
• 📚 Docs have been added/updated following the documentation template (for bug fixes / features)
• 🖼️ Validated PR Screenshots Compare Test Run results.
• ❗ Contains NO breaking changes
• 👀 Reviewed 2 other open pull requests (optional but appreciated!)