To debug TGUI interfaces with browser-style developer tools, there exists a utility that Microsoft bundles with Windows called IEChooser/F12 to debug any Internet Explorer/Trident-using interface, which BYOND uses.
This provides invaluable tools such as a local console, a DOM viewer, an interactive debugger, and more.
You can access the IEChooser.exe by pressing Win + R, then typing f12, then pressing enter. To manually go there: 64-bit version that we use is located at %windir%\SysWOW64\F12\IEChooser.exe. There’s also a 32-bit one in system32.
Simply launch the application after you’ve opened a TGUI window, and choose the .html name. This is likely to be something like tgui-window-1. There’s a refresh button in the top right.
Unfortunately, it seems this program doesn’t have a new target chooser if your window is fully closed so you’ll need to restart it if it disconnects from the window.
tgui-window-1.html is the thing