Make selections vertically tighter.

Introduce a new Row::contents_dim(), which height is restricted to the
row contents and does not contain any extra vertical spacing.

Rely on this for painting selections: if the selection starts on row,
for example, the ascent of the contents dim is considered. If ot was
started above, then the full dim is used.

Fixes bug #3899.

src/Row.h

@@ -210,6 +210,11 @@ public:
 	///
 	int descent() const { return dim_.des; }
 
+	///
+	Dimension const & contents_dim() const { return contents_dim_; }
+	///
+	Dimension & contents_dim() { return contents_dim_; }
+
 	/// The offset of the left-most cursor position on the row
 	int left_x() const;
 	/// The offset of the right-most cursor position on the row
@@ -348,6 +353,8 @@ private:
 	bool flushed_;
 	/// Row dimension.
 	Dimension dim_;
+	/// Row contents dimension. Does not contain the space above/below row.
+	Dimension contents_dim_;
 	/// true when this row lives in a right-to-left paragraph
 	bool rtl_;
 	/// true when a changebar should be drawn in the margin

src/RowPainter.cpp

@@ -577,8 +577,10 @@ void RowPainter::paintSelection() const
 	if (!row_.selection())
 		return;
 
-	int const y1 = yo_ - row_.ascent();
-	int const y2 = y1 + row_.height();
+	int const y1 = yo_ - (row_.begin_margin_sel ? row_.ascent()
+	                                            : row_.contents_dim().asc);
+	int const y2 = yo_ + (row_.end_margin_sel ? row_.descent()
+	                                          : row_.contents_dim().des);
 
 	// draw the margins
 	if (row_.isRTL() ? row_.end_margin_sel : row_.begin_margin_sel)

src/TextMetrics.cpp

@@ -1185,6 +1185,9 @@ void TextMetrics::setRowHeight(Row & row) const
 
 	row.dim().asc = maxasc;
 	row.dim().des = maxdes;
+
+	// This is useful for selections
+	row.contents_dim() = row.dim();
 }