1. start the game.

2.invite buddy.

3. pause game and shift+tab to turn on steam message

4. turn off steam message, comeback to the game and resume game

5. then keyboard control is limited.
move keys are changed into 4 arrow from wasd

can't swap weapon(space)

can't pick up new weapon(e)

all i can do is moving(using arrow keys), firing and using ability(left-right click).

-after quiting and restarting the game, it cameback to normal.

but when i turn on steam chat(shift+tab), then the problem occurs again.

how can i fix this problem(so that i can freely chat)?

This is some system-specific issue with Steam, you can search "keyboard does not work after closing steam overlay" to find a whole bunch of results across many different games.

Solutions also vary - for some people pressing Shift again helps, for others switching to other window and back helps. This is not a "stable" problem.