Select tool defaults to "subtract" mode, with a minus internal: 2.8.14p2,
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Using 2.8.14p2, I find the cursor selection often but not always to "subtract" mode. The lasso for example has a "-" on it.
I find I must hold the shift key to get into "add" mode to select anything.
2.8.14p2 is crashing frequently also, on a stock OS X Yosemite. It crashes most often when loading an image.
Possible dupe: https://sourceforge.net/p/gimponosx/bugs/68/