Skip to content

Fix ComboBox dropdown height after clearing or removing all items #14632

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
JayashreeSF3546 wants to merge 7 commits into
base: main
Choose a base branch
from
+44 −7
Open

Fix ComboBox dropdown height after clearing or removing all items #14632

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
7 commits
Select commit Hold shift + click to select a range
4f4549c
339 The height of the "DropDown" list for the combobox control can't …
JayashreeSF3546 Jun 4, 2026
a763143
added calculate height method
JayashreeSF3546 Jun 5, 2026
707852c
review changes
JayashreeSF3546 Jun 5, 2026
9f33e17
added test cases
JayashreeSF3546 Jun 10, 2026
67bec0e
review changes
JayashreeSF3546 Jun 10, 2026
cd0c96f
review changes
JayashreeSF3546 Jun 10, 2026
780acb4
removed unit test cases
JayashreeSF3546 Jun 17, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
30 changes: 30 additions & 0 deletions ...indows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.ComboBoxChildNativeWindow.cs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,18 @@ protected override unsafe void WndProc(ref Message m)
case PInvokeCore.WM_GETOBJECT:
WmGetObject(ref m);
return;
case PInvokeCore.WM_WINDOWPOSCHANGING:
if (_childWindowType == ChildWindowType.DropDownList)
{
WmWindowPosChanging(ref m);
DefWndProc(ref m);
}
else
{
_owner.ChildWndProc(ref m);
}

break;
Comment on lines +28 to +39
case PInvokeCore.WM_MOUSEMOVE:
if (_childWindowType == ChildWindowType.DropDownList)
{
Expand Down Expand Up @@ -153,5 +165,23 @@ private unsafe void WmGetObject(ref Message m)
throw new InvalidOperationException(SR.RichControlLresult, e);
}
}

private unsafe void WmWindowPosChanging(ref Message m)
{
// The native ComboBox sizes the dropdown list during its own layout
// pass (after CBN_DROPDOWN fires). Intercept here to enforce the
// managed computed height before the OS commits the final size.
// This ensures the UI reflects Items.Count (or explicit DropDownHeight)
// even when the list is empty or items are cleared at runtime.
WINDOWPOS* pos = (WINDOWPOS*)(nint)m.LParamInternal;
if (pos is not null && (pos->flags & SET_WINDOW_POS_FLAGS.SWP_NOSIZE) == 0)
{
int height = _owner.GetCalculatedDropDownHeight();
if (pos->cy != height)
{
pos->cy = height;
}
Comment on lines +177 to +183

@LeafShi1 LeafShi1 Jun 17, 2026

Copy link
Copy Markdown
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

  • The WM_WINDOWPOSCHANGING handler unconditionally overrides the dropdown height for every position-change event (as long as SWP_NOSIZE is not set):

     if (pos->cy != height)
 {
     pos->cy = height;
 }

    This means if the OS intentionally adjusts the dropdown height for legitimate reasons (DPI changes, integral height adjustments, display-edge clamping, or monitor work-area boundaries), the fix will forcefully override it. A safer approach would be to limit the intercept to specific scenarios — e.g., only when the dropdown is initially opening or when items have changed.

  • If the dropdown is near the bottom of the screen, the OS may reduce the dropdown height to fit within the work area. The current fix would override that clamped value back to the full calculated height, potentially causing the dropdown to extend off-screen. The fix should respect pos->cy when it's smaller than the calculated height if the dropdown position is near a screen edge.

}
}
}
}
21 changes: 14 additions & 7 deletions src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.cs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -3327,14 +3327,8 @@ public override string ToString()
return $"{s}, Items.Count: {_itemsCollection?.Count ?? 0}";
}

private void UpdateDropDownHeight()
private int GetCalculatedDropDownHeight()
{
if (_dropDownHandle.IsNull)
{
return;
}

// Now use the DropDownHeight property instead of calculating the Height...
int height = DropDownHeight;
if (height == DefaultDropDownHeight)
{
Expand All @@ -3343,6 +3337,19 @@ private void UpdateDropDownHeight()
height = ItemHeight * count + 2;
}

return height;
}

private void UpdateDropDownHeight()
{
if (_dropDownHandle.IsNull)
{
return;
}

// Now use the DropDownHeight property instead of calculating the Height...
int height = GetCalculatedDropDownHeight();

PInvoke.SetWindowPos(
_dropDownHandle,
HWND.HWND_TOP,
Expand Down
Loading