Add test for StyledText with fixed font metrics

This commit adds a test for StyledText with fixed font metrics that led to a regression with eclipse-platform#1610

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_custom_StyledText.java

@@ -63,6 +63,8 @@
 import org.eclipse.swt.graphics.Color;
 import org.eclipse.swt.graphics.Font;
 import org.eclipse.swt.graphics.FontData;
+import org.eclipse.swt.graphics.FontMetrics;
+import org.eclipse.swt.graphics.GC;
 import org.eclipse.swt.graphics.GlyphMetrics;
 import org.eclipse.swt.graphics.Point;
 import org.eclipse.swt.graphics.RGB;
@@ -5934,4 +5936,22 @@ public void test_rangeSelectionKeepsCaret() {
 	assertEquals("Selection does not start from caret", initialOffset, text.getSelection().x);
 	assertNotEquals("Selection is not left-to-right", text.getSelection().x, text.getCaretOffset());
 }
+
+@Test
+public void test_bug1610_fixedLineHeightWithChangingToSmallerFont_noException() {
+	shell.setVisible(true);
+	shell.setLayout(new GridLayout(1, false));
+
+	GC gc = new GC(shell.getDisplay());
+	FontMetrics metrics = gc.getFontMetrics();
+	text.setFixedLineMetrics(metrics);
+
+	FontData fontData = text.getFont().getFontData()[0];
+	text.setText("");
+	int smallFontHeight = metrics.getAscent() + metrics.getDescent() - 4;
+	Font font = new Font(text.getDisplay(), fontData.getName(), smallFontHeight, fontData.getStyle());
+	text.setFont(font);
+	font.dispose();
+	gc.dispose();
+}
 }