Also minor thing: the size of the level seems to be slightly bigger than the actual game window, so on the right and bottom side, a tina bit of level is not shown. not enough to affect gameplay as you can still see at least the bottom half of the character square and the enemy squares, but yeah.
gotta adjust that width and height setting for the game window there a bit.
also am playing on 1920x1080 resolution if that info is needed for clarification :-)
The game is full of bugs, but I playtested it many times and never encountered the ones you describe.
The purple enemy in the jump power-up room is resistant to normal bullets, you have to beat the first boss to acquire the charged shot and kill it :)
As for the size of the window, there is a delta of 10 pixels. I don't get why, but this seems to be a Java thing. Instantiating an x * y window causes a x+10 * y+10 window to appear. I had to account for it.