In a single browser window, Google Cloud Shell only lets me have one instance that is not corrupt. Screenshots explain. Is there something one can do using Firefox 57 Quantum on Windows 10 or Firefox ESR 52.5.0 on CentOS6 to have multiple google cloud shells that are actually useable? Resizing text in one "tab" may fix corruption in that tab at the cost of corrrupting the other tab.
You can try running Cloud Shell in safe mode by appending
cloudshellsafemode=true to the URL.
Sometimes running browsers in incognito mode helps prevent some browser extensions to cause unintended behavior.