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Fix issue 14795: ComboBox with DropDownStyle.Simple renders incorrectly and displays an unexpected vertical scrollbar #14827

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Fix issue 14795: ComboBox with DropDownStyle.Simple renders incorrectly and displays an unexpected vertical scrollbar #14827

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c4444e3
Fix issue 14795: ComboBox with DropDownStyle.Simple renders incorrect…
Jul 30, 2026
9a784e0
Handle feedback
Jul 31, 2026
5bacf62
Handle feedback
Aug 3, 2026
443dd66
Fix test case feedback
Aug 3, 2026
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251 changes: 209 additions & 42 deletions src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.Modern.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,10 @@ public partial class ComboBox
private bool _nativeComboHandleInitialized;
private bool _normalizingNativeComboBaseline;
private int _modernComboLayoutWriteCount;
private HWND _modernSimpleListClipRegionHandle;
private Size _modernSimpleListClipRegionSize;
private int _modernSimpleListClipRegionApplyCount;
private const int ModernSimpleBottomCropLogicalPixels = 2;

internal bool UsesModernComboAdapter
=> EffectiveVisualStylesMode >= VisualStylesMode.Net11
Expand Down Expand Up @@ -165,19 +169,6 @@ private ModernComboTargetState ComputeModernComboTargetState()
{
int topInset = chromeInsets.Top + Padding.Top;
int bottomInset = chromeInsets.Bottom + Padding.Bottom;
editBounds.Y += topInset;

editBounds.Height = Math.Max(
1,
editBounds.Height - topInset - bottomInset);

editBounds.Height = Math.Max(
1,
Math.Min(
editBounds.Height,
ClientRectangle.Bottom
- bottomInset
- editBounds.Top));

// Inset the edit window horizontally so its rectangular corners clear the rounded
// field arcs, and reserve the (now wider) drop-down button on the button side.
Expand All @@ -189,30 +180,88 @@ private ModernComboTargetState ComputeModernComboTargetState()
ModernControlVisualStyles.ComboBoxButtonExtraWidth,
DeviceDpiInternal);

if (RightToLeft == RightToLeft.Yes)
if (DropDownStyle == ComboBoxStyle.Simple)
{
// In RTL the drop-down button sits on the left, so its reservation goes there.
editBounds.X += rightInset + extraButtonWidth;
int dividerThickness = GetModernSimpleDividerThickness();
int simpleBottomShrink = ScaleHelper.ScaleToDpi(
ModernSimpleBottomCropLogicalPixels,
DeviceDpiInternal);
int simpleListBottom = ClientRectangle.Bottom - simpleBottomShrink - dividerThickness;
int simpleEditTop = topInset + ScaleHelper.ScaleToDpi(1, DeviceDpiInternal);
int selectionFieldHeight = Math.Max(
1,
(int)PInvokeCore.SendMessage(
this,
PInvoke.CB_GETITEMHEIGHT,
(WPARAM)(-1)));
int minimumReadableHeight = Math.Max(
1,
FontHeight + ScaleHelper.ScaleToDpi(2, DeviceDpiInternal));
int preferredSimpleEditHeight = Math.Max(selectionFieldHeight, minimumReadableHeight)
+ ScaleHelper.ScaleToDpi(2, DeviceDpiInternal);
int maxSimpleEditHeight = Math.Max(
1,
simpleListBottom - dividerThickness - simpleEditTop);

// Keep the editor in the top field lane with enough height for full glyph rendering,
// draw an accent divider below it, and let the permanent list consume the remaining
// space down to the rounded field border.
editBounds.Y = simpleEditTop;
editBounds.Height = Math.Min(preferredSimpleEditHeight, maxSimpleEditHeight);
editBounds.X = leftInset;
editBounds.Width = Math.Max(
1,
ClientRectangle.Width - leftInset - rightInset);

if (!simpleListBounds.IsEmpty)
{
simpleListBounds.X = editBounds.Left;
simpleListBounds.Width = editBounds.Width;
simpleListBounds.Y = editBounds.Bottom + dividerThickness;
simpleListBounds.Height = Math.Max(
1,
simpleListBottom - simpleListBounds.Y);
}
}
else
{
editBounds.X += leftInset;
}

editBounds.Width = Math.Max(
1,
editBounds.Width - leftInset - rightInset - extraButtonWidth);
editBounds.Y += topInset;

if (!simpleListBounds.IsEmpty)
{
int simpleListBottom = GetCurrentSimpleListBottom(
simpleListBounds.Bottom);

simpleListBounds.Y = editBounds.Bottom + bottomInset;
editBounds.Height = Math.Max(
1,
editBounds.Height - topInset - bottomInset);

simpleListBounds.Height = Math.Max(
editBounds.Height = Math.Max(
1,
Math.Min(
editBounds.Height,
ClientRectangle.Bottom
- bottomInset
- editBounds.Top));

if (RightToLeft == RightToLeft.Yes)
{
// In RTL the drop-down button sits on the left, so its reservation goes there.
editBounds.X += rightInset + extraButtonWidth;
}
else
{
editBounds.X += leftInset;
}

editBounds.Width = Math.Max(
1,
simpleListBottom - simpleListBounds.Y);
editBounds.Width - leftInset - rightInset - extraButtonWidth);

if (!simpleListBounds.IsEmpty)
{
int simpleListBottom = GetCurrentSimpleListBottom(
simpleListBounds.Bottom);
simpleListBounds.Y = editBounds.Bottom + bottomInset;
simpleListBounds.Height = Math.Max(
1,
simpleListBottom - simpleListBounds.Y);
}
}
}

Expand Down Expand Up @@ -281,6 +330,60 @@ private void GetAdjustedNativeChildBounds(
}
}

private void ConfigureModernSimpleListSurface(HWND listHandle)
{
if (DropDownStyle != ComboBoxStyle.Simple
|| !UsesModernComboAdapter
|| listHandle.IsNull)
{
return;
}

WINDOW_STYLE style = (WINDOW_STYLE)PInvokeCore.GetWindowLong(
listHandle,
WINDOW_LONG_PTR_INDEX.GWL_STYLE);
WINDOW_EX_STYLE exStyle = (WINDOW_EX_STYLE)PInvokeCore.GetWindowLong(
listHandle,
WINDOW_LONG_PTR_INDEX.GWL_EXSTYLE);

WINDOW_STYLE updatedStyle = style & ~WINDOW_STYLE.WS_BORDER;
WINDOW_EX_STYLE updatedExStyle = exStyle & ~WINDOW_EX_STYLE.WS_EX_CLIENTEDGE;

if (updatedStyle == style && updatedExStyle == exStyle)
{
return;
}

if (updatedStyle != style)
{
PInvokeCore.SetWindowLong(
listHandle,
WINDOW_LONG_PTR_INDEX.GWL_STYLE,
(nint)updatedStyle);
}

if (updatedExStyle != exStyle)
{
PInvokeCore.SetWindowLong(
listHandle,
WINDOW_LONG_PTR_INDEX.GWL_EXSTYLE,
(nint)updatedExStyle);
}

PInvoke.SetWindowPos(
listHandle,
HWND.Null,
0,
0,
0,
0,
SET_WINDOW_POS_FLAGS.SWP_NOMOVE
| SET_WINDOW_POS_FLAGS.SWP_NOSIZE
| SET_WINDOW_POS_FLAGS.SWP_NOZORDER
| SET_WINDOW_POS_FLAGS.SWP_NOACTIVATE
| SET_WINDOW_POS_FLAGS.SWP_FRAMECHANGED);
}

private unsafe int GetCurrentSimpleListBottom(
int fallback)
{
Expand Down Expand Up @@ -366,6 +469,8 @@ private unsafe void ApplyModernComboLayout()
return;
}

ConfigureModernSimpleListSurface(comboBoxInfo.hwndList);

if (!target.EditBounds.IsEmpty)
{
ApplyEditMargins(
Expand Down Expand Up @@ -464,26 +569,85 @@ private void ApplyEditMargins(

private void ApplyChildBounds(HWND childHandle, Rectangle targetBounds)
{
bool isModernSimpleList = UsesModernComboAdapter
&& DropDownStyle == ComboBoxStyle.Simple
&& childHandle == _childListBox?.HWND;

if (childHandle.IsNull
|| targetBounds.IsEmpty
|| GetChildBounds(childHandle) == targetBounds)
|| targetBounds.IsEmpty)
{
return;
}

PInvoke.SetWindowPos(
childHandle,
HWND.Null,
targetBounds.Left,
targetBounds.Top,
targetBounds.Width,
targetBounds.Height,
SET_WINDOW_POS_FLAGS.SWP_NOACTIVATE
| SET_WINDOW_POS_FLAGS.SWP_NOZORDER);
if (GetChildBounds(childHandle) != targetBounds)
{
PInvoke.SetWindowPos(
childHandle,
HWND.Null,
targetBounds.Left,
targetBounds.Top,
targetBounds.Width,
targetBounds.Height,
SET_WINDOW_POS_FLAGS.SWP_NOACTIVATE
| SET_WINDOW_POS_FLAGS.SWP_NOZORDER);

_modernComboLayoutWriteCount++;
_modernComboLayoutWriteCount++;
}

if (isModernSimpleList)
{
ConfigureModernSimpleListClipRegion(
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childHandle,
targetBounds);
}
}

private void ConfigureModernSimpleListClipRegion(
HWND listHandle,
Rectangle targetBounds)
{
if (listHandle.IsNull)
{
return;
}

int bottomShrink = ScaleHelper.ScaleToDpi(
ModernSimpleBottomCropLogicalPixels,
DeviceDpiInternal);
int visibleHeight = Math.Max(1, targetBounds.Height - bottomShrink);
Size targetRegionSize = new(targetBounds.Width, visibleHeight);

if (listHandle == _modernSimpleListClipRegionHandle
&& targetRegionSize == _modernSimpleListClipRegionSize)
{
return;
}

using RegionScope region = new(
0,
0,
targetRegionSize.Width,
targetRegionSize.Height);

if (PInvoke.SetWindowRgn(
listHandle,
region,
fRedraw: true) != 0)
{
region.RelinquishOwnership();
_modernSimpleListClipRegionHandle = listHandle;
_modernSimpleListClipRegionSize = targetRegionSize;
_modernSimpleListClipRegionApplyCount++;
}
}

private int GetModernSimpleDividerThickness()
=> Math.Max(
1,
ScaleHelper.ScaleToDpi(
ModernControlVisualStyles.BorderThickness,
DeviceDpiInternal));

private Rectangle GetChildBounds(HWND child)
{
PInvokeCore.GetWindowRect(child, out RECT bounds);
Expand Down Expand Up @@ -526,6 +690,9 @@ private int GetNativeComboBaselineSelectionFieldItemHeight()
private int GetModernComboLayoutWriteCount()
=> _modernComboLayoutWriteCount;

private int GetModernSimpleListClipRegionApplyCount()
=> _modernSimpleListClipRegionApplyCount;

private Padding GetModernFieldPadding()
{
SystemVisualSettings settings = Application.SystemVisualSettings;
Expand Down
40 changes: 40 additions & 0 deletions ...ystem.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.ModernComboAdapter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,7 @@ public override void DrawFlatCombo(
graphics,
clientBounds);
DrawDropDownButton(comboBox, graphics);
DrawSimpleEditListDivider(comboBox, graphics);

switch (_flatStyle)
{
Expand Down Expand Up @@ -395,6 +396,45 @@ private static void CutOutRoundedCorners(
parentColor);
}

private static void DrawSimpleEditListDivider(
ComboBox comboBox,
Graphics graphics)
{
if (comboBox.DropDownStyle != ComboBoxStyle.Simple
|| comboBox._childListBox is null)
{
return;
}

Rectangle listBounds = comboBox.GetChildBounds(comboBox._childListBox.HWND);
if (listBounds.IsEmpty)
{
return;
}

int dividerThickness = Math.Max(
1,
ScaleHelper.ScaleToDpi(
ModernControlVisualStyles.BorderThickness,
comboBox.DeviceDpiInternal));
int lineY = listBounds.Top - dividerThickness;
if (lineY < 0)
{
return;
}

Padding chromeInsets = comboBox.GetModernChromeInsets();
int left = chromeInsets.Left + comboBox.Padding.Left;
int right = comboBox.ClientRectangle.Right - chromeInsets.Right - comboBox.Padding.Right - 1;
if (right < left)
{
return;
}

using var pen = Application.SystemVisualSettings.AccentColor.GetCachedPenScope(dividerThickness);
graphics.DrawLine(pen, left, lineY, right, lineY);
}

private static void DrawRoundedBorder(
ComboBox comboBox,
Graphics graphics,
Expand Down
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