Fix 13976: Enhance TreeNode.UpdateImage() to support inherited image keys and selected image rendering

[LeafShi1]

Fixes #13976

Proposed changes

• Add switch PreserveUnassignedTreeNodeImages to do following change
  • Update TreeNode.UpdateImage() by adding logic to resolve ImageKey and SelectedImageKey from TreeView when not explicitly set on the node.
  • Improved fallback handling for both image and selected image index:
    • Priority: ImageKey → ImageIndex → TreeView.ImageKey → TreeView.ImageIndex → default (0)
    • Same logic applied to SelectedImageKey and SelectedImageIndex
  • Prevented setting iSelectedImage when no valid image is available, allowing native fallback to iImage
  • Ensured TVITEMW.mask includes TVIF_SELECTEDIMAGE only when applicable

Customer Impact

• After this fix, when the TreeView’s ImageList changes, TreeNodes without explicitly set images will continue to display their original default image—if it still exists. If the default image has been removed, they will fall back to showing the first image in the new ImageList.

Regression?

• No

Risk

• Minimal

Screenshots

Example project: TreeViewImageBug-master.zip

Before

TreeNodes without an image set are changed to the first image if the ImageList is changed.

BeforeChanges.mp4

After

TreeNodes without an image set are changed to the first image only when the original image is removed

AfterChanges.mp4

Test methodology

• Manually

Test environment(s)

• .net 10.0.0-rc.1.25405.103

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR fixes an issue where TreeNodes without explicitly set images would incorrectly display the first image in a new ImageList when the TreeView's ImageList is changed. The fix implements a proper inheritance chain from TreeView to TreeNode for both regular and selected images, ensuring nodes maintain their intended images unless those images are removed from the new ImageList.

Key Changes:

• Added logic to resolve ImageKey and SelectedImageKey from TreeView when not explicitly set on the node
• Implemented a four-tier fallback hierarchy: Node ImageKey → Node ImageIndex → TreeView ImageKey → TreeView ImageIndex → default (0)
• Applied the same inheritance logic to both regular and selected images

[codecov]

Codecov Report

❌ Patch coverage is 27.27273% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.15035%. Comparing base (76554a2) to head (b2d5757).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13981         +/-   ##
===================================================
+ Coverage   77.14861%   77.15035%   +0.00173%     
===================================================
  Files           3276        3276                 
  Lines         645116      645161         +45     
  Branches       47705       47711          +6     
===================================================
+ Hits          497698      497744         +46     
- Misses        143727      143734          +7     
+ Partials        3691        3683          -8     

Flag	Coverage Δ
Debug	77.15035% <27.27273%> (+0.00173%)	⬆️
integration	18.99029% <0.00000%> (+0.01169%)	⬆️
production	52.01441% <27.27273%> (+0.00601%)	⬆️
test	97.40955% <ø> (+0.00007%)	⬆️
unit	49.45750% <27.27273%> (-0.00549%)	⬇️

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[ricardobossan]

All LGTM! Maybe add a test with the switch on and off, like done in https://github.com/ricardobossan/winforms/blob/7ccafe298bdf138f09585d3993c62768892f3d89/src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/AccessibleObjects/DataGridViewRowAccessibleObjectTests.cs#L2397-L2401

[LeafShi1]

All LGTM! Maybe add a test with the switch on and off

Waiting for @KlausLoeffelmann @JeremyKuhne to confirm this solution.

UpdateImage() sends a message to the Win32 API via SendMessage to update the UI, but does not change the ImageKey or ImageIndex properties of the TreeNode. Therefore, to verify whether the TreeNode displays the correct image on the UI, image rendering or screenshot comparison is required, which is not possible in Unit test.