Shadow on the PyCharm window

I am running PyCharm in Linux in WSL, and I use command line pycharm-professional & to launch PyCharm.

However, I can always see a dark colored shadow on the PyCharm window after I resize it.

Let's look into this example:

Can you see the dark block on the bottom horizontally across the Jupyter code block and the “Jupyter Variables” section?

Initially, this issue does not appear. But after I resize the PyCharm window to the size of the dark block, I see the whole window become dark as the block. Then after I maximize it, the area before maximizing is still as dark as before, but other area become light as normal.

Then we see this shadow.

This shadow also go with the PyCharm window. It stays there even if I drag the window into another monitor.