Here's a possible solution for anyone that has run into issues getting Gamestudio games to work properly on Windows tablets.

The problem: Touch events != mouse events. Mouse inputs will lag behind, UI elements will barely work at all, and trying to tap a button will actually tap whatever you touched before that.

The solution: Grab window events as they happen and make your own input system.

First things first, you need to define an event handler.

Code:
#define TOUCH_MESSAGE 581
#define RELEASE_MESSAGE 514
#define RELEASE2_MESSAGE 123
var touchMouseDown = 0;
VECTOR touchMousePos;
VECTOR touchMickey;

LRESULT CALLBACK baseMessageHandler(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);
LRESULT CALLBACK handleTouch(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
	
	if(message == TOUCH_MESSAGE)
	{ 
		VECTOR temp;
		getXYparam(lParam, temp);
		if(!touchMouseDown)
		{
			vec_set(touchMousePos, mouse_pos);
			vec_zero(touchMickey);
		}
		touchMouseDown = 1;
		
		if(touchEnable)
		{
			mouse_pos.x = temp.x;
			mouse_pos.y = temp.y;
		}
	}
	if(message == RELEASE_MESSAGE || message == RELEASE2_MESSAGE)
	{
		touchMouseDown = 0;
	}
	return baseMessageHandler(hwnd,message,wParam,lParam);   	  
}

void setMessageHandler_startup()
{
	baseMessageHandler = on_message;
	on_message = handleTouch;
}


Next, you may or may not need to break out the plugin API. If anyone has a lite-C alternative, feel free to provide that. Otherwise, we need this C++ function:

Code:
DLLFUNC void getXYparam(LPARAM in, VECTOR* out)
{
    out->x = _VAR(GET_X_LPARAM(in));
    out->y = _VAR(GET_Y_LPARAM(in));
}


Once we've finished that, here's where the magic happens. This is a portion of my main() loop, which supports both drag and tap. touchMickey is like the built-in mickey value but touch friendly, and can be used for panning the camera around. And we can fill in a function called uiMouseDown() that can do something useful with the touch position, such as seeking buttons to click. Note: This will NOT work with standard panel buttons and click events. You will need to iterate through clickable buttons manually and perform c_traces instead.

Code:
var mouse = 0;
	var mouseClick = 0;
	
	while(1)
	{		
		vec_diff(touchMickey, mouse_pos, touchMousePos);
		vec_set(touchMousePos, mouse_pos);
		
		if(touchMouseDown)// || mouse_left)
		{
			if(!mouseClick)
			{
				mouseClick = 1;
				uiMouseDown();
			}
			
			if(mouse > 2)
			{
				camera.pan -= touchMickey.x * 0.15;
				camera.tilt = clamp(camera.tilt - touchMickey.y * 0.15, -85, 85);
			}
			else
			{
				++mouse;
			}
		}
		else
		{
			mouseClick = 0;
			mouse = 0;
		} 
		wait(1);
	}


This is from a working demo, so if this example does not work, complain about it and I will have a look and see what I missed. Hope this helps somebody!

Another note: This has been tested only on a full-screen application. The mouse coordinates are given as absolute screen positions and would need to have the window client area position subtracted if you want to allow for windowed games.