Fix jitter when editing RTL text

When drawing right aligned text, rounding errors in the position
returned by GetDrawPosition() cause the right margin of the text to
change whenever text width changes causing "jumping letters" effect. So
here we calculate the drawing position relative to the right margin on
our own to avoid the rounding errors. That is basically a hack, and it
should go away if one day we managed to get rid of those rounding
errors.

Continue using GetDrawPosition() for non-right aligned text, to minimize
any unforeseen side effects.

Change-Id: Ia095a17a41f723f5fb7afc99df4250262e9a0051

1 files changed, 51 insertions, 15 deletions

diff --git a/vcl/coretext/ctlayout.cxx b/vcl/coretext/ctlayout.cxx
index 50ab4400ace9..dd3b2ea26d96 100644
--- a/vcl/coretext/ctlayout.cxx
+++ b/vcl/coretext/ctlayout.cxx
@@ -51,6 +51,9 @@ public:
     virtual void    Simplify( bool bIsBase );
 
 private:
+    CGPoint         GetTextDrawPosition(void) const;
+    double          GetWidth(void) const;
+
     const CTTextStyle* const    mpTextStyle;
 
     // CoreText specific objects
@@ -65,7 +68,7 @@ private:
 
     // x-offset relative to layout origin
     // currently only used in RTL-layouts
-    mutable long    mnBaseAdv;
+    mutable double  mfBaseAdv;
 };
 
 // =======================================================================
@@ -76,7 +79,7 @@ CTLayout::CTLayout( const CTTextStyle* pTextStyle )
 ,   mpCTLine( NULL )
 ,   mnCharCount( 0 )
 ,   mfCachedWidth( -1 )
-,   mnBaseAdv( 0 )
+,   mfBaseAdv( 0 )
 {
     CFRetain( mpTextStyle->GetStyleDict() );
 }
@@ -144,7 +147,7 @@ void CTLayout::AdjustLayout( ImplLayoutArgs& rArgs )
     // in RTL-layouts trailing spaces are leftmost
     // TODO: use BiDi-algorithm to thoroughly check this assumption
     if( rArgs.mnFlags & SAL_LAYOUT_BIDI_RTL)
-        mnBaseAdv = rint( fTrailingSpace );
+        mfBaseAdv = fTrailingSpace;
 
     // return early if there is nothing to do
     if( nPixelWidth <= 0 )
@@ -166,11 +169,42 @@ void CTLayout::AdjustLayout( ImplLayoutArgs& rArgs )
     }
     CFRelease( mpCTLine );
     mpCTLine = pNewCTLine;
-    mfCachedWidth = -1; // TODO: can we set it directly to target width we requested? For now we re-measure
+    mfCachedWidth = nPixelWidth;
 }
 
 // -----------------------------------------------------------------------
 
+// When drawing right aligned text, rounding errors in the position returned by
+// GetDrawPosition() cause the right margin of the text to change whenever text
+// width changes causing "jumping letters" effect. So here we calculate the
+// drawing position relative to the right margin on our own to avoid the
+// rounding errors. That is basically a hack, and it should go away if one day
+// we managed to get rid of those rounding errors.
+//
+// We continue using GetDrawPosition() for non-right aligned text, to minimize
+// any unforeseen side effects.
+CGPoint CTLayout::GetTextDrawPosition(void) const
+{
+    float fPosX, fPosY;
+
+    if (mnLayoutFlags & SAL_LAYOUT_RIGHT_ALIGN)
+    {
+        // text is always drawn at its leftmost point
+        const Point aPos = DrawBase();
+        fPosX = aPos.X() + mfBaseAdv - GetWidth();
+        fPosY = aPos.Y();
+    }
+    else
+    {
+        const Point aPos = GetDrawPosition(Point(mfBaseAdv, 0));
+        fPosX = aPos.X();
+        fPosY = aPos.Y();
+    }
+
+    CGPoint aTextPos = { +fPosX, -fPosY };
+    return aTextPos;
+}
+
 void CTLayout::DrawText( SalGraphics& rGraphics ) const
 {
     AquaSalGraphics& rAquaGraphics = static_cast<AquaSalGraphics&>(rGraphics);
@@ -188,8 +222,7 @@ void CTLayout::DrawText( SalGraphics& rGraphics ) const
     CGContextSetShouldAntialias( rAquaGraphics.mrContext, !rAquaGraphics.mbNonAntialiasedText );
 
     // Draw the text
-    const Point aVclPos = GetDrawPosition( Point(mnBaseAdv,0) );
-    CGPoint aTextPos = { (CGFloat) +aVclPos.X(), (CGFloat) -aVclPos.Y() };
+    CGPoint aTextPos = GetTextDrawPosition();
 
     if( mpTextStyle->mfFontRotation != 0.0 )
     {
@@ -328,7 +361,7 @@ int CTLayout::GetNextGlyphs( int nLen, sal_GlyphId* pGlyphIDs, Point& rPos, int&
 
 // -----------------------------------------------------------------------
 
-long CTLayout::GetTextWidth() const
+double CTLayout::GetWidth() const
 {
     if( (mnCharCount <= 0) || !mpCTLine )
         return 0;
@@ -337,10 +370,13 @@ long CTLayout::GetTextWidth() const
         mfCachedWidth = CTLineGetTypographicBounds( mpCTLine, NULL, NULL, NULL);
     }
 
-    const long nScaledWidth = lrint( mfCachedWidth );
-    return nScaledWidth;
+    return mfCachedWidth;
 }
 
+long CTLayout::GetTextWidth() const
+{
+    return lrint(GetWidth());
+}
 // -----------------------------------------------------------------------
 
 long CTLayout::FillDXArray( sal_Int32* pDXArray ) const
@@ -441,8 +477,8 @@ bool CTLayout::GetBoundRect( SalGraphics& rGraphics, Rectangle& rVCLRect ) const
     CGContextScaleCTM( rAquaGraphics.mrContext, 1.0, -1.0 );
     CGContextSetShouldAntialias( rAquaGraphics.mrContext, !rAquaGraphics.mbNonAntialiasedText );
 
-    const Point aVclPos = GetDrawPosition( Point(mnBaseAdv,0) );
-    CGPoint aTextPos = { (CGFloat) +aVclPos.X(), (CGFloat) -aVclPos.Y() };
+    const CGPoint aVclPos = GetTextDrawPosition();
+    CGPoint aTextPos = GetTextDrawPosition();
 
     if( mpTextStyle->mfFontRotation != 0.0 )
     {
@@ -465,10 +501,10 @@ bool CTLayout::GetBoundRect( SalGraphics& rGraphics, Rectangle& rVCLRect ) const
 
     CGContextRestoreGState( rAquaGraphics.mrContext );
 
-    rVCLRect.Left()   = aVclPos.X() + aMacRect.origin.x;
-    rVCLRect.Right()  = aVclPos.X() + (aMacRect.origin.x + aMacRect.size.width);
-    rVCLRect.Bottom() = aVclPos.Y() - (aMacRect.origin.y);
-    rVCLRect.Top()    = aVclPos.Y() - (aMacRect.origin.y + aMacRect.size.height);
+    rVCLRect.Left()   = aVclPos.x + aMacRect.origin.x;
+    rVCLRect.Right()  = aVclPos.x + (aMacRect.origin.x + aMacRect.size.width);
+    rVCLRect.Bottom() = aVclPos.x - (aMacRect.origin.y);
+    rVCLRect.Top()    = aVclPos.x - (aMacRect.origin.y + aMacRect.size.height);
 
     return true;
 }