The issue of clicking into a window and the input being submitted to another window that is behind it, has been reported in this forum a few times already (here’s one and I remember that I’ve seen two more but I can’t locate them right now).
There are few bug tickets about this behavior but I think the best one to track is this: 478556 – With NVIDIA or AMD GPUs on X11, sometimes left or right clicking on IntelliJ IDE windows or a Plasma panel instead interacts with Plasma desktop
Also, its a good idea to check if you have the “slide back” Desktop Effect enabled - it is know to cause problem with the window stacking order (which is this bug).