I have the same problem.
Right ALT should NOT be used for menu. You can’t reproduce problem because you does not understand it.
Normally when using right alt for special letters like “ś, ć, ż, ź, etc.” menu does not popup, it happens only if you type too fast.
Look, to type “ś” you need to click “alt + s”, but if you type fast you often unintentionally release alt" before pressing “s”. Now you have a problem, because “alt” opened menu and next typed letters started to navigate throught the menu.
This problem drives me crazy, often closes new-message window and once sended my unfinished message to customs, .
Check word “właściwy” for example, and try to unintentionally release “alt” before pressing “s”: w, alt+l, a, alt, s, c, i, w … and the end, before word ends your message will be sent, because “s, c, i, w” will navigate to Message>Send.
Please, exlude right alt from triggering menu.