editor: Optimize LineNumberArea painting for large documents

block.isVisible() is always True, check the rectangle to find out
if it is really in the viewport.

Break the loop if the block is below the viewport.

Move some common variables outside the loop.

Refs #360.

ReText/editor.py

@@ -457,21 +457,25 @@ def paintEvent(self, event):
 			return QWidget.paintEvent(self, event)
 		painter = QPainter(self)
 		painter.fillRect(event.rect(), colorValues['lineNumberArea'])
+		painter.setPen(colorValues['lineNumberAreaText'])
 		cursor = QTextCursor(self.editor.document())
 		cursor.movePosition(QTextCursor.Start)
 		atEnd = False
+		fontHeight = self.fontMetrics().height()
+		height = self.editor.height()
 		if globalSettings.relativeLineNumbers:
 			relativeTo = self.editor.textCursor().blockNumber()
 		else:
 			relativeTo = -1
 		while not atEnd:
 			rect = self.editor.cursorRect(cursor)
 			block = cursor.block()
-			if block.isVisible():
+			if rect.top() >= height:
+				break
+			elif rect.bottom() > 0:
 				number = str(cursor.blockNumber() - relativeTo).replace('-', '−')
-				painter.setPen(colorValues['lineNumberAreaText'])
 				painter.drawText(0, rect.top(), self.width() - 2,
-					self.fontMetrics().height(), Qt.AlignRight, number)
+				                 fontHeight, Qt.AlignRight, number)
 			cursor.movePosition(QTextCursor.EndOfBlock)
 			atEnd = cursor.atEnd()
 			if not atEnd: