Fixing a WPF Tray ContextMenu That Does Not Close on Focus Loss

Overview

This article addresses an issue observed while developing TreePaste where a tray icon right-click menu remained visible until a menu item was selected.
The resolution combines ContextMenu.StaysOpen = false with Win32Api.SetForegroundWindow() immediately before opening the menu.

Prerequisites / Environment

Framework / Language: .NET 10 / C# 14
UI stack: WPF with System.Windows.Forms.NotifyIcon
Architecture: code-behind
Target project: TreePaste

Problem

When the tray icon was right-clicked, a WPF ContextMenu opened as expected.
However, the menu did not close when focus moved away, and stayed visible until a ContextMenuItem was clicked.
This behavior degraded the expected tray-menu interaction model.

Cause / Background

A WPF ContextMenu close behavior depends on both menu configuration and window activation context.
Tray-origin interactions do not always behave like standard in-window right-click flows, so focus-transition signals can become inconsistent.
In that state, opening a menu without foreground alignment can leave close-on-focus-loss behavior unreliable.

Solution

The adopted solution uses two coordinated steps.

Set ContextMenu.StaysOpen = false.
Call Win32Api.SetForegroundWindow() right before ContextMenu.IsOpen = true.

This combination aligns activation state at menu-open time and restores expected close behavior when focus moves elsewhere.

Implementation

First, call SetForegroundWindow in the tray right-click handler before opening the menu.
The following snippet is based on the implementation used in TreePaste MainWindow.xaml.cs.

_notifyIcon.MouseClick += (_, e) =>
{
    if (e.Button == System.Windows.Forms.MouseButtons.Right)
    {
        Dispatcher.Invoke(() =>
        {
            var helper = new WindowInteropHelper(this);
            Win32Api.SetForegroundWindow(helper.Handle);
            _trayContextMenu.Placement = System.Windows.Controls.Primitives.PlacementMode.MousePoint;
            _trayContextMenu.IsOpen = true;
        });
    }
};

This execution order normalizes foreground state first, then opens the menu.
Opening with only IsOpen = true tends to be less stable for tray-origin focus transitions.

Next, configure the tray menu to close on outside interaction.
The following snippet is from TreePaste CreateTrayContextMenu().

return new System.Windows.Controls.ContextMenu
{
    Items = { showItem, githubItem, separator, exitItem },
    StaysOpen = false
};

StaysOpen = false is necessary but may be insufficient on its own in tray scenarios.
Using it together with foreground alignment provides stable behavior in practice.

SetForegroundWindow is defined in TreePaste Win32Api as follows.

[DllImport("user32.dll")]
public static extern bool SetForegroundWindow(IntPtr hWnd);

This P/Invoke bridge allows the WPF app to coordinate tray menu activation with Win32 window state.

Notes

SetForegroundWindow is still subject to Windows foreground restrictions and should not be treated as an unconditional override.
For mixed NotifyIcon + WPF ContextMenu implementations, menu operations should remain on the UI thread via Dispatcher.Invoke.
StaysOpen = false alone may not fully resolve close behavior if the opening window context is not foreground-aligned.

Alternatives / Comparison

Approach	Benefits	Drawbacks	Best fit
`StaysOpen = false` only	Minimal implementation	Can still fail in tray-origin scenarios	Standard in-window context menus
`SetForegroundWindow` only	Improves activation context	Does not replace menu close policy	Cases where menu config is fixed
Use both (adopted)	Stabilizes both open-state and close-state behavior	Adds Win32 interop dependency	Tray context menus requiring consistent UX

Summary

For the TreePaste tray menu issue, combining StaysOpen = false and SetForegroundWindow() resolved the persistent menu display problem.
The result is a context menu that opens in a foreground-aligned state and closes when focus moves away, matching standard user expectations.
For WPF tray menus, designing both menu policy and foreground transition together is the most reliable approach.