In windows if mailspring is minimized from the minimize button to the taskbar, then hidden in tray by clicking on the tray icon, and then restored to taskbar by click on tray icon again, then click on the mailspring in the task bar to maximize it, now if you try to click inside mailsrping nothing is responsive.
If I use close (Taskbar icon gone) and open Mailspring again through the tray icon the interface works normal.
Also, when you minimized, one way to get it open responsive again is to right-click the tray icon and select preferences. After Mailspring opened the preference page just go back to the inbox, user interface is responsive.
The issue is quite annoying, happens several times a day.
The issue is still occuring on Windows 10 with Mailspring Version 1.13.3
Opening Mailspring from tray leads to a black Mailspring window appearing. When clicking the tray icon again, the black Mailspring window closes . Then clicking the tray icon yet again finally opens the Mailspring window properly. (I.e. the same workaround as @ForNeVeR described)