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1. I have 2 Swamps and 1 Forest on battefield
2. I cast Grizzly Bears ( ). UI says "Mana left to play: 1 G" - it's ok
3. I tap Swamp: UI still says "Mana left to play: 1 G" - shouldn't it be just G?

next:
1. Cast any spell (why not again Grizzly Bears)
2. Tap two Swamps for mana
3. Press cancel, Swamp untaps, but UI shows two colorless mana in your mana pool.

first issue:
I will fix that. It happens because LM detects that some colored mana is missing and doesn't even try colorless. this is clearly misleading

second issue:
I hope the mana was black? the mana pool labels were not yet changed to PropertyChangeListeners, so they are not updated when you cancel. The mana pool is still correct; try doing it again and you will see that the next update restores the correct nujmber and doesn't add to 3