UI layout update delay when increasing item count (switching from lower to higher counts) #11294
Description
Description
When increasing the number of items displayed in a WPF ListBox from a lower count to a higher count, there is a noticeable delay before the UI updates and the layout stabilizes. The control becomes momentarily unresponsive during the transition, and the final layout takes additional time to render. This is especially visible when using grid-like layouts (e.g., WrapPanel or UniformGrid) and higher row/column density.
ScreenCapture_12-1-2025.6.19.44.PM.mp4
Reproduction Steps
1.Run the attached sample application.
2.Click the "Load Items" button to load the initial set of items (count = 50).
Note: The control loads these items in less than a second (time shown in the dialog).
3.Using the ComboBox, select 50 × 50 to load 2,500 items.
4.Observe that the ListBox control takes significantly more time to update the items and render the layout.
The total time taken is approximately 10 seconds, and sometimes it can take up to 20 seconds.
Expected behavior
We expect the control to take only a few seconds to update the layout, including rendering, even when switching to a higher item count.
Actual behavior
The control takes around 10 seconds, and sometimes more than 20 seconds, to update the layout and render the items when switching to a higher item count.
Regression?
No response
Known Workarounds
No response
Impact
No response
Configuration
OS: Windows 11
.NET / .NET Framework: .NET Framework 4.8
UI Framework: WPF
Other information
No response