Could be interesting to add the need of typing of the correct word to kill the incorrect ones (or to make a powered hit)?
Something like Kratos' (God-of-War) or Kenshirō (PS1's Hokuto no Ken: Seiki Matsukyu Seishi Densetsu) final-killing hits.
The same problem happens to me using the browser version (without zoom) in Google Chrome.
I'm using it with Google Chrome (v. 79.0.3945.117 (off. Build, 64 bit), JavaScript V8 7.9.317.32 and Flash 32.0.0.31 (with chrome://version) with auto-updates) on Win7-Pro-64.
[I know it's old... I'm upgrading PC and OS now]
The same problem happens to me using the Windows .jar (1.0) version AND maximize-ing the window.
With default window size it works.
I'm using it with Win7-Pro-64 and javaw.exe "Java(TM) Platform SE binary" (in [...]\Java\jdk1.8.0_172\bin\javaw.exe ; using «java.exe -version» it tells «Java(TM) SE Runtime Environment (build 1.8.0_172-b11)») wich is "Java 8 update 172" (and "Java 8u172", see: oracle.com/technetwork/java/javase/jdk8-naming-2157130.html) it should be from 20180417 (java.com/it/download/faq/release_dates.xml).
[I know it's all old... I'm upgrading PC and OS now]