This is terribly annoying. But there are workarounds mentioned on https://developercommunity2.visualstudio.com/t/Unexpected-VS-crash-when-docking-or-spli/1323017:

I tried this one and it helps:

Edit %InstallRoot%\Common7\IDE\devenv.exe.config and %LocalAppData%\Microsoft\VisualStudio\16.0_xxx\devenv.exe.config and append the following text to the element’s value attribute:

;Switch.System.Windows.Interop.MouseInput.OptOutOfMoveToChromedWindowFix=true;
Switch.System.Windows.Interop.MouseInput.DoNotOptOutOfMoveToChromedWindowFix=true

All of this text must be put into one line without spaces in between. The line break in this post is only for readability.