Avoid using DTM_GETIDEALSIZE with DTS_SHOWNONE

This message is completely broken when DTS_SHOWNONE is used, it returns
wrong result (less than the size without DTS_SHOWNONE) initially and
completely wrong results after a DPI change.

Create a temporary date time picker control without DTS_SHOWNONE and
call DTM_GETIDEALSIZE for it instead. This is wasteful, but at least
returns correct results.

This commit is best viewed ignoring whitespace-only changes.

src/msw/datetimectrl.cpp

@@ -164,24 +164,66 @@ wxSize wxDateTimePickerCtrl::DoGetBestSize() const
 {
     wxSize size;
 
-    // We can ask the control itself to compute its ideal size, but we only use
+    // Use DTM_GETIDEALSIZE to ask the control itself to compute its ideal size.
-    // it for the horizontal component: the vertical size is not computed
+    SIZE idealSize = { 0, 0 };
-    // correctly after the DPI of the window has changed because for every DPI
+    if ( wxGetWinVersion() >= wxWinVersion_Vista )
-    // change, the returned size is 4 pixels higher, even if the DPI is
-    // lowered, so we always need to compute it ourselves below.
-    //
-    // Also work around https://bugs.winehq.org/show_bug.cgi?id=44680 by
-    // checking for the return value: even if all "real" MSW systems do support
-    // this message, Wine does not, even when it's configured to return Vista
-    // or later version to the application, and returns FALSE for it.
-    SIZE idealSize;
-    if ( wxGetWinVersion() >= wxWinVersion_Vista
-            && ::SendMessage(m_hWnd, DTM_GETIDEALSIZE, 0, (LPARAM)&idealSize) )
     {
-        size.x = idealSize.cx;
+        // We can't use DTM_GETIDEALSIZE with DTS_SHOWNONE because handling of
-        size.y = GetCharHeight();
+        // this flag is completely broken (up to at least Window 10 20H2): it's
+        // not just ignored, but we get completely wrong results when this flag
+        // is on, e.g. the returned width is less than the width without it or
+        // much greater than the real value after a DPI change (and growing
+        // with every new change, even when repeatedly switching between the
+        // same DPI values, e.g. dragging a window between 2 monitors with
+        // different scaling). Moreover, note that even without DTS_SHOWNONE,
+        // DTM_GETIDEALSIZE still returns wrong results for the height after a
+        // DPI change, so we never use the vertical component of the value
+        // returned by it.
+        //
+        // Unfortunately, resetting this style doesn't work neither, so we have
+        // to create a whole new window just for this, which is pretty wasteful
+        // but seems unavoidable.
+        HWND hwnd;
+        if ( MSWAllowsNone() )
+        {
+            hwnd = ::CreateWindow
+                     (
+                        DATETIMEPICK_CLASS,
+                        wxT(""),
+                        ::GetWindowLong(GetHwnd(), GWL_STYLE) & ~DTS_SHOWNONE,
+                        0, 0, 1, 1,
+                        GetHwndOf(m_parent),
+                        0,
+                        wxGetInstance(),
+                        NULL
+                     );
+            wxCHECK_MSG( hwnd, wxSize(),
+                         wxS("SysDateTimePick32 creation unexpected failed") );
+
+            wxSetWindowFont(hwnd, GetFont());
+        }
+        else
+        {
+            hwnd = GetHwnd();
+        }
+
+        // Also work around https://bugs.winehq.org/show_bug.cgi?id=44680 by
+        // checking for the return value: even if all "real" MSW systems do support
+        // this message, Wine does not, even when it's configured to return Vista
+        // or later version to the application, and returns FALSE for it.
+        if ( ::SendMessage(hwnd, DTM_GETIDEALSIZE, 0, (LPARAM)&idealSize) )
+        {
+            size.x = idealSize.cx;
+            size.y = GetCharHeight();
+        }
+
+        if ( hwnd != GetHwnd() )
+        {
+            ::DestroyWindow(hwnd);
+        }
     }
-    else // Compute the size ourselves.
+
+    if ( !idealSize.cx ) // Compute the size ourselves.
     {
         // Use the same native format as the underlying native control.
 #if wxUSE_INTL
@@ -202,16 +244,13 @@ wxSize wxDateTimePickerCtrl::DoGetBestSize() const
         size.x += wxSystemSettings::GetMetric(wxSYS_HSCROLL_ARROW_X, m_parent);
     }
 
-    // We need to account for the checkbox, if we have one -- DTM_GETIDEALSIZE
+    // Account for the checkbox.
-    // doesn't take it into account (actually, it somehow returns _smaller_
-    // size when using DTS_SHOWNONE than when not using it, which doesn't make
-    // any sense at all).
     if ( MSWAllowsNone() )
     {
-        // The extra 10px here was determined heuristically as the value which
+        // The extra 8px here was determined heuristically as the value which
         // results in the same layout with and without DTS_SHOWNONE under
         // Windows 7 and Windows 10 with 100%, 150% and 200% scaling.
-        size.x += wxGetSystemMetrics(SM_CXMENUCHECK, m_parent) + 10;
+        size.x += wxGetSystemMetrics(SM_CXMENUCHECK, m_parent) + 8;
     }
 
     int scrollY = wxSystemSettings::GetMetric(wxSYS_HSCROLL_ARROW_Y, m_parent);