Fix text jumps during selection dragging on MSW

When selecting the text in wxHtmlWindow by dragging the mouse the text
to the right of the selection sometimes jumps horizontally. This happens
only on MSW and only for TrueType fonts. The reason is that
wxDC::GetPartialTextExtents() and wxDC::GetTextExtent() give different
results if the text contains characters with underhangs or overhangs.

Fix this by caching results of wxDC::GetPartialTextExtents() and using
them instead of calling wxDC::GetTextExtent().

include/wx/html/htmlcell.h

@@ -35,7 +35,8 @@ public:
     wxHtmlSelection()
         : m_fromPos(wxDefaultPosition), m_toPos(wxDefaultPosition),
           m_fromCharacterPos(-1), m_toCharacterPos(-1),
-          m_fromCell(NULL), m_toCell(NULL) {}
+          m_fromCell(NULL), m_toCell(NULL),
+          m_extBeforeSel(0), m_extBeforeSelEnd(0) {}
 
     // this version is used for the user selection defined with the mouse
     void Set(const wxPoint& fromPos, const wxHtmlCell *fromCell,
@@ -58,6 +59,11 @@ public:
     wxCoord GetFromCharacterPos () const { return m_fromCharacterPos; }
     wxCoord GetToCharacterPos () const { return m_toCharacterPos; }
 
+    void SetExtentBeforeSelection(unsigned ext) { m_extBeforeSel = ext; }
+    void SetExtentBeforeSelectionEnd(unsigned ext) { m_extBeforeSelEnd = ext; }
+    unsigned GetExtentBeforeSelection() const { return m_extBeforeSel; }
+    unsigned GetExtentBeforeSelectionEnd() const { return m_extBeforeSelEnd; }
+
     bool IsEmpty() const
         { return m_fromPos == wxDefaultPosition &&
                  m_toPos == wxDefaultPosition; }
@@ -66,6 +72,12 @@ private:
     wxPoint m_fromPos, m_toPos;
     wxCoord m_fromCharacterPos, m_toCharacterPos;
     const wxHtmlCell *m_fromCell, *m_toCell;
+
+    // Extent of the text before selection start.
+    unsigned m_extBeforeSel;
+
+    // Extent of the text from the beginning to the selection end.
+    unsigned m_extBeforeSelEnd;
 };
 
 
@@ -431,7 +443,8 @@ protected:
     void SetSelectionPrivPos(const wxDC& dc, wxHtmlSelection *s) const;
     void Split(const wxDC& dc,
                const wxPoint& selFrom, const wxPoint& selTo,
-               unsigned& pos1, unsigned& pos2) const;
+               unsigned& pos1, unsigned& pos2,
+               unsigned& ext1, unsigned& ext2) const;
 
     wxString m_Word;
     bool     m_allowLinebreak;

src/html/htmlcell.cpp

@@ -343,7 +343,8 @@ void wxHtmlWordCell::SetPreviousWord(wxHtmlWordCell *cell)
 // where s2 and s3 start:
 void wxHtmlWordCell::Split(const wxDC& dc,
                            const wxPoint& selFrom, const wxPoint& selTo,
-                           unsigned& pos1, unsigned& pos2) const
+                           unsigned& pos1, unsigned& pos2,
+                           unsigned& ext1, unsigned& ext2) const
 {
     wxPoint pt1 = (selFrom == wxDefaultPosition) ?
                    wxDefaultPosition : selFrom - GetAbsPos();
@@ -400,21 +401,30 @@ void wxHtmlWordCell::Split(const wxDC& dc,
     pos2 = j;
 
     wxASSERT( pos2 >= pos1 );
+
+    ext1 = pos1 == 0 ? 0 : (pos1 < widths.size() ? widths[pos1-1] : widths.Last());
+    ext2 = pos2 == 0 ? 0 : (pos2 < widths.size() ? widths[pos2-1] : widths.Last());
 }
 
 void wxHtmlWordCell::SetSelectionPrivPos(const wxDC& dc, wxHtmlSelection *s) const
 {
-    unsigned p1, p2;
+    unsigned p1, p2, ext1, ext2;
 
     Split(dc,
           this == s->GetFromCell() ? s->GetFromPos() : wxDefaultPosition,
           this == s->GetToCell() ? s->GetToPos() : wxDefaultPosition,
-          p1, p2);
+          p1, p2, ext1, ext2);
 
     if ( this == s->GetFromCell() )
+    {
         s->SetFromCharacterPos(p1); // selection starts here
+        s->SetExtentBeforeSelection(ext1);
+    }
     if ( this == s->GetToCell() )
+    {
         s->SetToCharacterPos(p2); // selection ends here
+        s->SetExtentBeforeSelectionEnd(ext2);
+    }
 }
 
 
@@ -459,7 +469,6 @@ void wxHtmlWordCell::Draw(wxDC& dc, int x, int y,
         // Selection changing, we must draw the word piecewise:
         wxHtmlSelection *s = info.GetSelection();
         wxString txt;
-        int w, h;
         int ofs = 0;
 
         // NB: this is quite a hack: in order to compute selection boundaries
@@ -479,8 +488,7 @@ void wxHtmlWordCell::Draw(wxDC& dc, int x, int y,
         {
             txt = m_Word.Mid(0, part1);
             dc.DrawText(txt, x + m_PosX, y + m_PosY);
-            dc.GetTextExtent(txt, &w, &h);
+            ofs += s->GetExtentBeforeSelection();
-            ofs += w;
         }
 
         SwitchSelState(dc, info, true);
@@ -490,11 +498,9 @@ void wxHtmlWordCell::Draw(wxDC& dc, int x, int y,
 
         if ( (size_t)part2 < m_Word.length() )
         {
-            dc.GetTextExtent(txt, &w, &h);
-            ofs += w;
             SwitchSelState(dc, info, false);
             txt = m_Word.Mid(part2);
-            dc.DrawText(txt, ofs + x + m_PosX, y + m_PosY);
+            dc.DrawText(txt, x + m_PosX + s->GetExtentBeforeSelectionEnd(), y + m_PosY);
         }
         else
             drawSelectionAfterCell = true;