Synchronisation problem in Line Input

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I am using the Calc Spreadsheet on a Macbook 10.6.8. I am typing in text (wordwrapped) in a cell often with several lines. If I use the delete key or use the left arrow in the line input window (the small window above the main cells window), its position on the type varies - either right(correct), centre or left. This seems to be a bit random. It can become difficult to delete a single letter because I am never sure which letter will be deleted. Supposing I type these numbers - 123456. Normally I would click to the right of 4 to delete it. Sometimes I have to click in the middle of 5 or on the right side of 5 to delete 4. All very confusing. I am using "courier" as I thought maybe a fixed font spacing may help. Any ideas you have would be appreciated. Thanks . Maurice.