Ticket 520287: SfComboBox.MaxDropDownHeight issue.

I'm using SfComboBox to provide a dropdown 'attached' to a button.

• The ComboBox is positioned at -1 ZIndex
• IsEditable/IsDropdownButtonVisible are set to false.
• All Color properties set to transparent.
• The button's action toggles SfComboBox.IsDropdownOpen

The goal is to ensure the height of the dropdown is set to the height of the contents.

To accomplish this I use the following logic:

• The items in the content use a set of well-known item templates
• SfComboBox.ItemTemplate is set to DataTemplateSelector that returns the appropriate DataTemplate
• The 'container' defines a hidden content view for each DataTemplate
  • These are referred to as 'placeholders' and are to measure the content height.

I use a custom type for the ItemsSource that pre-calculates the number of each 'item type' in the collection as well as the specific item type that has the 'largest' size. The BindingContext of the placeholder items is updated.

• Subscribe to SfComboBox.PropertyChanging and filter for IsDropdownOpen)
• Subscribe to SfComboBox.PropertyChanged and filter for ItemsSource
• On ItemsSource changed:
  • Set a cached MaxDropDownHeight value to zero to force a recalculation.
• On IsDropDownOpen is changing:
  • Calculate MaxDropdownHeight if zero
  • Set SComboBox.MaxDropDownHeight to the cached value.

At this point, everything works as expected. The dropdown always reflects the calculated size.

One usage of the ComboBox is to present an MRU of recently opened files. In my application, this is where the repro occurs.

• Subscribe to SfComboBox.SelectedItem changed
• Within SelectedItem changed:
  • Create a new dropdown list with the selected item to the start of the list.
  • Update SfComboBox.ItemsSource to the new list.
  • At this point, MaxDropdownHeight continues to behave as expected.

Since I could not reproduce the issue in the sample application, I updated my application's dropdown logic to mirror this implementation and expected the to see the application behave correctly but the issue still occurs.

After comparing the two implementations in detail, I observed only one difference. The sample application uses the default font size in the various templates while the actual application explicitly sets the font sizes.

I updated the sample application to use an explicit font size of 20 in the various templates and now the issue occurs, the MaxDropDownHeight is set correctly. Tracing before and after the setting MaxDropDownHeight shows the expected value; however, the size of the actual popup is too small.

It 'appears' the issue likely occurs when using a non-default or perhaps 'larger' font size is used. There may be simpler scenarios where this occurs but I focused on my use-case.

The sample application populates the dropdown with a set of directories and files found at the root of c:. If you are not running on Windows or do not have a 'C' drive, change the const string DefaultPath in MainViewModel.cs.

• NOTE: The sample does not have a proper layout class so the initial dropdown button size is incorrect. Click the button twice to fix.

There are two actions available:

• Select the 'Refresh Dropdown Contents' button to simply refresh the dropdown contents.

• Select the button above the 'refresh' button to display the dropdown and select one of the items. This invokes the 'SelectedItem' logic.

I first assumed that updating the ItemsSource within SelectedItem changed caused the problem; however, the problem occurs in both cases, simply refreshing and refreshing within SelectedItem changed.

If you remove the following line from MainPage.xaml, the repro no longer occurs:

<Setter Property="FontSize" Value="20"/>

The following shows the dropdown before refresh:

And after refresh: