Win32 tooltip gray line bug in Windows 10 - winapi

Question

We have been using code that creates classical Win32 multiline tooltips in our legacy VB6 component for many years, since the times of Windows XP. It works fine in all latest versions of MS Windows (7, 8.1) except Windows 10. A parasitic horizontal gray line appears in the tooltip in this OS. The best demonstration of this problem is a tooltip window containing several lines of text (the main tip text is multiline and/or the tooltip has a bold title):
The correct tooltip should look like this (a screen from Windows 8.1):
Below is one more example of the same problem when the tooltip window does not have tile/icon but contains only multiline text:
This parasitic gray line is also present in a single-line tooltip - though it is not noticeable at first look:
What it could be? Is it a bug in Windows 10, or something has changed in the tooltip API?
Below is the code of the method used to initialize a tooltip:
Public Function Create(ByVal ParentHwnd As Long) As Boolean
Dim lWinStyle As Long
If m_lTTHwnd <> 0 Then
DestroyWindow m_lTTHwnd
End If
m_lParentHwnd = ParentHwnd
lWinStyle = TTS_ALWAYSTIP Or TTS_NOPREFIX
m_lTTHwnd = CreateWindowExA(0&, _
TOOLTIPS_CLASS, _
vbNullString, _
lWinStyle, _
CW_USEDEFAULT, _
CW_USEDEFAULT, _
CW_USEDEFAULT, _
CW_USEDEFAULT, _
0&, _
0&, _
App.hInstance, _
0&)
'now set our tooltip info structure
Dim tiA As TOOLINFOA
Dim tiW As TOOLINFOW
If g_bIsNt Then
With tiW
.lSize = Len(tiW)
.lFlags = TTF_SUBCLASS Or TTF_IDISHWND
.hWnd = m_lParentHwnd
.lId = m_lParentHwnd '0
.hInstance = App.hInstance
.lpStr = StrPtr(mvarTipText)
End With
Else
With tiA
.lSize = Len(tiA)
.lFlags = TTF_SUBCLASS Or TTF_IDISHWND
.hWnd = m_lParentHwnd
.lId = m_lParentHwnd
.hInstance = App.hInstance
.lpStr = mvarTipText
End With
End If
'add the tooltip structure
If g_bIsNt Then
SendMessage m_lTTHwnd, TTM_ADDTOOLW, 0&, tiW
Else
SendMessage m_lTTHwnd, TTM_ADDTOOLA, 0&, tiA
End If
'if we want a title or we want an icon
If mvarTitle <> vbNullString Or mvarIcon <> igToolTipIconNone Then
If g_bIsNt Then
SendMessage m_lTTHwnd, TTM_SETTITLEW, mvarIcon, ByVal StrPtr(mvarTitle)
Else
SendMessage m_lTTHwnd, TTM_SETTITLEA, mvarIcon, ByVal mvarTitle
End If
End If
' set the time parameters
SendMessageByLongA m_lTTHwnd, TTM_SETDELAYTIME, TTDT_AUTOPOP, mvarVisibleTime
SendMessageByLongA m_lTTHwnd, TTM_SETDELAYTIME, TTDT_INITIAL, mvarDelayTime
'according to MSDN, we should set TTM_SETMAXTIPWIDTH to a positive value
'to enable multiline tooltips
SendMessageByLongA m_lTTHwnd, TTM_SETMAXTIPWIDTH, 0, 100000
End Function

Answer 1

To solve the problem, we should not set the hwnd field of the TOOLINFO structure. The corresponding part of the code should look like this:
'now set our tooltip info structure
Dim tiA As TOOLINFOA
Dim tiW As TOOLINFOW
If g_bIsNt Then
With tiW
.lSize = Len(tiW)
.lFlags = TTF_SUBCLASS Or TTF_IDISHWND
.lId = m_lParentHwnd
.hInstance = App.hInstance
.lpStr = StrPtr(mvarTipText)
End With
Else
With tiA
.lSize = Len(tiA)
.lFlags = TTF_SUBCLASS Or TTF_IDISHWND
.lId = m_lParentHwnd
.hInstance = App.hInstance
.lpStr = mvarTipText
End With
End If