Mouse Clicks
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Button 1: Sets the cursor, drawing it in the new location, and marks the clicked vertex.
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Button 2: Sets the cursor without drawing it in the new location and unmarks the clicked vertex.
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Button 3: Clears marked vertices.
 

Mouse Controls for Zooming
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Control+Button 1: Zooms in, doubling the zoom level
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Control+Button 3: Zooms out, halving the zoom level