Keys stuck after Alt-Tab
Why a character keeps moving after Alt-Tab, and the quickest order of fixes.
The symptom is easy to recognise: the character keeps moving in one direction after the fingers have stopped pressing. Sometimes it stops on its own after a moment, sometimes it continues until the round ends. In both cases the cause is usually not the game but a key release that never reached the page. Understanding that cause saves a lot of time, because the fix depends on it rather than on luck or on reloading everything.
A browser sends key-down and key-up events to whichever page holds focus. When you switch windows with Alt-Tab, the old window loses focus at once, and the key-up event for a key still held down may never be sent. The page then believes the key is still pressed, because the only message that would tell it otherwise never arrived. This happens in every browser, and it is not a bug in any particular game.
The first fix is to click the play area, then press and release each movement key once, one after another. The click restores focus, and each complete press-and-release gives the page the event it missed. Do it for W, A, S, D and the arrow keys even if you only use some of them. It takes about two seconds and usually settles the problem without reloading anything or losing the round you are in.
The better approach is prevention. Before Alt-Tab, release every movement key and wait a moment. Do not hold W while pressing Alt-Tab, because that combination produces stuck keys more often than any other. If you need to switch windows regularly, use the pause button in the game first, then switch. A paused round is not sending events that can go missing, so there is nothing for the page to misread while you are away.
On Windows, two settings make this worse. Sticky Keys turns on after five presses of Shift, and Filter Keys ignores brief or repeated presses. Both live in Settings, Accessibility, Keyboard. Turn both off if you do not need them. Filter Keys in particular can make a game feel as though its controls are broken, because fast presses are filtered out before they ever reach the page, and the delay looks like lag rather than a setting.
Full screen reduces how often this happens. In full screen the address bar and toolbar are gone, so there is no other area to click by accident and no reason to switch windows. If you must check something outside the game, leave full screen first, release the keys, then switch. The order matters; switching first and releasing afterwards does not help, because the page has already lost the events by the time your fingers come off the keys.
If keys are still stuck after all of that, reload the tab. This is the last step because it throws away the round in progress, and in a room of two to four players the loss of one player mid-round is felt by everybody. Reload once, not repeatedly. After the reload, rejoin with the same nickname and wait for the next round to begin rather than trying to force your way into the one already running.
Finally, get into the habit of checking three things before each session: full screen is on, no other window is competing for attention, and both Sticky Keys and Filter Keys are off. Those three checks take less than ten seconds, and they remove almost every stuck key that would otherwise happen in the middle of a round. A small problem avoided is worth more than a small problem fixed over and over.