The splitter area is very small, only 4 pixels, as such, we see that it is indeed very hard to hit this with touch. From a UI and UX perspective, I think that for touch, it is better to show a handle/button that can open/close the left panel for example.
I also noticed that the component TWebSplitter does not work on a tablet browser (Android / iPad). That means that the splitter cannot be moved with one finger.
Unfortunately I don't have an iPad or Android tablet with a pencil. Would it work with that?
This appears to be a non-trivial issue given all this is done via touch events on a mobile device.
We implement a first step for improvement with allowing a touch click to select the splitter and then permit to move the splitter. We'll see for future developments if we can further refine this.