I honestly don't know if there is a standardized way to pass the information or if it depends on the brand. I remember in the earlier version of gnome I had on my lenovo yoga the visual keyboard would popup on most text box but not on firefox which obliged me tolo use Gnome Web / epiphany when in tablet mode but they sorted it out later. I think there wasn't a tablet mode per se but I had used gnome tweak or an extension to have the accesibility options easy to access and enable the visual keyboard but it may have been mpre automatic later. I am saying all this out of memory because that computer died 2 years ago and I didn't use the tablet mode enough to replace it with similar one.