This was a very fun game (bought every single upgrade to confirm there were no hidden upgrades left).
Some small bugs that I found (except for the mushroom taking over my hole screen, other people have mentioned it) are:
1: when I fully bought the click upgrades, the game lagged for a very short time each time I clicked. Probably something to do with the amount of mushrooms and the calculations on the backend (maybe its checking every single mushroom if it is contained in the circle, and because of the big scaling it lags the game), I naturally can't access the backend, but from my perspective that sounds like the most reasonable explanation.
2: There are some upgrades with numbers on them showing the back-end values (like growth speed), but personally I would suggest rewriting those into seconds:miliseconds. (this is just one calculation on the backend, but makes it more clear for the end user). Also the gold chance is kinda wierd with it's value, so people can get confused because it shows 100 even tho it doesn't have a 100% chance of being gold.
Very fun like previously stated, with small bugs that don't really ruin the game, but I would recommend fixing them, as these could have some major effects lateron (more as in fixing them lateron, as code layered on code becomes unsearchable).
Short list of bugs (this includes both mine, and just general ones):
1: ui is hidden behind mushrooms
2: way to many mushrooms to reasonable collect with both the collectors and your mouse
3: hidden upgrades
4: game lags when clicking cursor when having the cursor upgrades
5: numbers in upgrades unclear
These are minor bugs, some of them fixable with only a few lines of code, others a bit more difficult, but they all look pretty easyish to fix.
Gl with the game, and I hope the features get added (that other guy had great ideas, but I believe you can also come up with some great ones, as you already made this)
Bye
edit:
Forgot about the targetting thing from the collectors, it's kinda wierd with 500+ mushrooms that they still chose to ignore the close ones, the oldest one targetting is I think 100% the reason, but what you could do is make set positions for mushrooms, keep these values known, and put a counter from 0-1 that saves if there is a mushroom there or not, then when a collector searches for the next mushroom, it looks for the closest position values around it, and if one of those is 1, it goes there, checks again and repeats, if multiple of those are 1, it goes trough this process (I think in java but im not sure):
Math.floor(Math.random*x)
x is the amount of them closeby
this generates a mostly random number between each of them, and goes to that one, this fixes most of the bugs this new thing could create, and fixes the most annoying part of the game.
Bye, once again.