Fix Issue 14721: TreeView.CustomDraw leaks one HFONT per visible item per paint when NodeFont is set, leading to GDI handle exhaustion and crash

[SimonZhao888]

Fixes #14721

Root Cause

TreeView.CustomDraw calls GetItemRenderStyles during every ITEMPREPAINT event, and this method creates a new OwnerDrawPropertyBag. The original code utilized renderinfo.FontHandle, which triggered Font.ToHfont()—allocating a new HFONT—whenever the property was first accessed in a new bag. Because these short-lived objects could not be released immediately, GDI font handles accumulated rapidly during high-frequency repainting, eventually leading to handle exhaustion and a crash.

Proposed changes

• The source for selecting the font into the Device Context has been changed from renderinfo.FontHandle to node._propBag.FontHandle, utilizing the reusable, long-lived cache available on the TreeNode. Null checks (for node._propBag and node._propBag.Font) are retained to ensure safety. This change preserves standard rendering semantics (using the same font source) while avoiding the repeated creation of an HFONT during every paint operation.
• In OwnerDrawPropertyBag, change Font from an auto-property to one with a custom setter: when the font changes, explicitly dispose of the old _fontWrapper and set it to null, ensuring that the correct HFONT is recreated the next time FontHandle is accessed.

Customer Impact

• Avoid exceptions (such as Win32Exception: The operation completed successfully.) and subsequent application crashes (0xC000041D) caused by reaching the 10,000 GDI handle limit.
• Reduce or eliminate the rapid proliferation of GDI font handles during frequent TreeView repainting.
• After switching TreeNode.NodeFont at runtime, the new font is correctly reflected, and the old HFONT is not used.

Regression?

• No

Risk

• Mini

Screenshots

Before

14721-Before.mp4

After

14721-After.mp4

Change the font of a TreeNode:

14721-After2.mp4

Test methodology

• Manually

Test environment(s)

• 11.0.100-preview.5.26302.115

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR addresses a GDI handle leak in TreeView.CustomDraw by avoiding per-paint HFONT creation when TreeNode.NodeFont is set, reducing the risk of GDI handle exhaustion and process crashes during frequent repaint scenarios.

Changes:

• Switches the selected font handle in the custom draw path from a short-lived OwnerDrawPropertyBag instance to the node’s long-lived _propBag cache.
• Adds guards/asserts around _propBag usage when selecting the font into the item DC.

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[LeafShi1]

ontHandle  is internal and directly accessible in the test assembly (InternalsVisibleTo), so the reflection lookup for it is unnecessary

[LeafShi1]

Consider using TestAccessor.Dynamic instead of reflection.

[LeafShi1]

The node._propBag.Font is not null part is redundant with the higher-level check renderinfo.Font is not null. GetItemRenderStyles sets retval.Font = node._propBag.Font (same reference) and returns an empty bag (Font == null) whenever node/node._propBag is null, so once renderinfo.Font is not null holds, node._propBag.Font is guaranteed non-null too and this inner check can be dropped. (Keep the node._propBag is not null half only to satisfy the compiler's nullable analysis, which can't infer that invariant across the method boundary.)