Implement 0-width pen consistently in wxGraphicsContext

Emulate a 1-pixel pen width as closely as possible.

This reverts:
334cf1cc91 (Take HiDPI scale into account for wxGCDC 0-width pen, 2021-04-03)
0d80050057 (Make wxGCDC behavior with 0-width wxPen consistent with MSW wxDC, 2021-03-02)

See #19077, #19115

src/generic/graphicc.cpp

@@ -441,17 +441,23 @@ public:
 
     virtual bool ShouldOffset() const wxOVERRIDE
     {
-        if ( !m_enableOffset )
+        if (!m_enableOffset || m_pen.IsNull())
             return false;
 
-        int penwidth = 0 ;
-        if ( !m_pen.IsNull() )
-        {
-            penwidth = (int)((wxCairoPenData*)m_pen.GetRefData())->GetWidth();
-            if ( penwidth == 0 )
-                penwidth = 1;
-        }
-        return ( penwidth % 2 ) == 1;
+        const double width = static_cast<wxCairoPenData*>(m_pen.GetRefData())->GetWidth();
+
+        // always offset for 1-pixel width
+        if (width <= 0)
+            return true;
+
+        // no offset if overall scale is not odd integer
+        double x = GetContentScaleFactor(), y = x;
+        cairo_user_to_device_distance(m_context, &x, &y);
+        if (!wxIsSameDouble(fmod(wxMin(fabs(x), fabs(y)), 2.0), 1.0))
+            return false;
+
+        // offset if pen width is odd integer
+        return wxIsSameDouble(fmod(width, 2.0), 1.0);
     }
 
     virtual void Clip( const wxRegion &region ) wxOVERRIDE;
@@ -844,8 +850,6 @@ wxCairoPenData::wxCairoPenData( wxGraphicsRenderer* renderer, const wxGraphicsPe
 {
     Init();
     m_width = info.GetWidth();
-    if (m_width <= 0.0)
-        m_width = 0.1;
 
     switch ( info.GetCap() )
     {
@@ -1017,7 +1021,14 @@ void wxCairoPenData::Apply( wxGraphicsContext* context )
     wxCairoPenBrushBaseData::Apply(context);
 
     cairo_t * ctext = (cairo_t*) context->GetNativeContext();
-    cairo_set_line_width(ctext,m_width);
+    double width = m_width;
+    if (width <= 0)
+    {
+        double x = context->GetContentScaleFactor(), y = x;
+        cairo_user_to_device_distance(ctext, &x, &y);
+        width = 1 / wxMin(fabs(x), fabs(y));
+    }
+    cairo_set_line_width(ctext, width);
     cairo_set_line_cap(ctext,m_cap);
     cairo_set_line_join(ctext,m_join);
     cairo_set_dash(ctext, m_lengths, m_count, 0);
@@ -1912,21 +1923,26 @@ wxCairoBitmapData::~wxCairoBitmapData()
 class wxCairoOffsetHelper
 {
 public :
-    wxCairoOffsetHelper( cairo_t* ctx , bool offset )
+    wxCairoOffsetHelper(cairo_t* ctx, double scaleFactor, bool offset)
     {
         m_ctx = ctx;
-        m_offset = offset;
-        if ( m_offset )
-             cairo_translate( m_ctx, 0.5, 0.5 );
+        m_offset = 0;
+        if (offset)
+        {
+             double x = scaleFactor, y = x;
+             cairo_user_to_device_distance(ctx, &x, &y);
+             m_offset = 0.5 / wxMin(fabs(x), fabs(y));
+             cairo_translate(m_ctx, m_offset, m_offset);
+        }
     }
     ~wxCairoOffsetHelper( )
     {
-        if ( m_offset )
-            cairo_translate( m_ctx, -0.5, -0.5 );
+        if (m_offset > 0)
+            cairo_translate(m_ctx, -m_offset, -m_offset);
     }
-public :
+private:
     cairo_t* m_ctx;
-    bool m_offset;
+    double m_offset;
 } ;
 
 #if wxUSE_PRINTING_ARCHITECTURE
@@ -1976,7 +1992,7 @@ wxCairoContext::wxCairoContext( wxGraphicsRenderer* renderer, const wxWindowDC&
     m_width = width;
     m_height = height;
 
-    m_enableOffset = dc.GetContentScaleFactor() <= 1;
+    EnableOffset();
 
 #ifdef __WXMSW__
     HDC hdc = (HDC)dc.GetHDC();
@@ -2029,7 +2045,7 @@ wxCairoContext::wxCairoContext( wxGraphicsRenderer* renderer, const wxMemoryDC&
     m_width = width;
     m_height = height;
 
-    m_enableOffset = dc.GetContentScaleFactor() <= 1;
+    SetContentScaleFactor(dc.GetContentScaleFactor());
 
 #ifdef __WXMSW__
     wxBitmap bmp = dc.GetSelectedBitmap();
@@ -2352,7 +2368,7 @@ wxCairoContext::wxCairoContext(wxGraphicsRenderer* renderer, HWND hWnd)
 {
     // See remarks for wxWindowBase::GetContentScaleFactor
     double scaleY = ::GetDeviceCaps((HDC)m_mswWindowHDC, LOGPIXELSY) / 96.0f;
-    m_enableOffset = scaleY <= 1.0;
+    SetContentScaleFactor(scaleY);
 
     m_mswStateSavedDC = 0;
     m_mswSurface = cairo_win32_surface_create((HDC)m_mswWindowHDC);
@@ -2393,7 +2409,7 @@ wxCairoContext::wxCairoContext( wxGraphicsRenderer* renderer, wxWindow *window)
     , m_mswWindowHDC(GetHwndOf(window))
 #endif
 {
-    m_enableOffset = window->GetContentScaleFactor() <= 1;
+    EnableOffset();
 #ifdef __WXGTK__
     // something along these lines (copied from dcclient)
 
@@ -2605,7 +2621,7 @@ void wxCairoContext::StrokePath( const wxGraphicsPath& path )
 {
     if ( !m_pen.IsNull() )
     {
-        wxCairoOffsetHelper helper( m_context, ShouldOffset() ) ;
+        wxCairoOffsetHelper helper(m_context, GetContentScaleFactor(), ShouldOffset());
         cairo_path_t* cp = (cairo_path_t*) path.GetNativePath() ;
         cairo_append_path(m_context,cp);
         ((wxCairoPenData*)m_pen.GetRefData())->Apply(this);
@@ -2618,7 +2634,7 @@ void wxCairoContext::FillPath( const wxGraphicsPath& path , wxPolygonFillMode fi
 {
     if ( !m_brush.IsNull() )
     {
-        wxCairoOffsetHelper helper( m_context, ShouldOffset() ) ;
+        wxCairoOffsetHelper helper(m_context, GetContentScaleFactor(), ShouldOffset());
         cairo_path_t* cp = (cairo_path_t*) path.GetNativePath() ;
         cairo_append_path(m_context,cp);
         ((wxCairoBrushData*)m_brush.GetRefData())->Apply(this);
@@ -2647,7 +2663,7 @@ void wxCairoContext::DrawRectangle( wxDouble x, wxDouble y, wxDouble w, wxDouble
     }
     if ( !m_pen.IsNull() )
     {
-        wxCairoOffsetHelper helper( m_context, ShouldOffset() ) ;
+        wxCairoOffsetHelper helper(m_context, GetContentScaleFactor(), ShouldOffset());
         ((wxCairoPenData*)m_pen.GetRefData())->Apply(this);
         cairo_rectangle(m_context, x, y, w, h);
         cairo_stroke(m_context);