1: Put an item into the hands slot, an item sufficiently big enough so that the item is in the middle of the screen (where the mouse will be when the inventory is opened).
2: Open inventory.
3: Now the inspect screen should open on top of the inventory, without pressing any other button than the inventory button.

This bug only works on items that are big enough for the mouse to hover over it the instant the inventory is opened, which means that I have only reproduced the bug with items such as clothing, backpacks and vests.
This bug can be prevented once if the item has been put into your hands by holding interact button on it to put it into your hands. But only once, then the bug returns, this can also be reproduced every time. So if you run around and drop and pickup whatever is in your hands, you can effectively mitigate the bug. Although it rather annoying and time consuming.

I have also seen that whenever I leave a server and return to the main menu, my character is locked to the mouse without clicking anything, so I can rotate him without pressing any keys or buttons, might or might not have something to do with it.