Vimium / Vimium C's modal indicator is currently drawn via the DOM similarly to how link hints are drawn. This presents some issues with certain pages, one of the most common examples being YouTube, as Insert Mode is required to use default YouTube shortcuts, yet fullscreening the browser continues to draw the insert mode indicator.

While it may be possible to avoid drawing modal indicators in scenarios like this by detecting a fullscreen window, I think the cleanest solution is extension icons for modal indicators, especially since Insert Mode is effectively a passthrough mode.

When using Vimium C, it makes sense that DOM manipulations need to occur to draw link hints etc. But when in Insert Mode, you intend not to use Vimium; therefore I think requiring no DOM manipulation would be best in this scenario. It also seems easier to maintain while avoiding conflicts.