Simply:

if( mouse.x == 0 )    scroll.x -= value;else if( mouse.x == SCREEN_WIDTH-1 )    scroll.x += value;if( mouse.y == 0 )    scroll.y -= value;else( mouse.y == SCREEN_HEIGHT-1 )    scroll.y += value;

The scene scrolls with a constant speed no matter how "fast" you try to move the mouse.
You don't have to handle the outer positions at all. If you scroll like this in an RTS, you can see that the mouse never leaves the client area, it "sticks" on it's borders.
If you are in windowed mode, it doesn't matter if the positions aren't updated, because you have to scroll with constant speed, even if you don't move the mouse at all, after you reached the edges (like in any RTS scrolling.)

Or if you have some kind of controls, menus at the edges, than you have to define areas, where you want the scrolling to happen. If you have menu on the top, and other controls on the bottom for example:

if( mouse.y > bottom_scroll_control_area.bottom    && mouse.y < bottom_scroll_control_area.top )    scroll.y -= value;else if( mouse.y > top_scroll_control_area.bottom    && mouse.y < top_scroll_control_area.top )    scroll.y += value;

These areas are say 10 pixels "tall".

This all stuff requires, that you handle the scrolling in your game loop, not in the window message thing, the scrolling will be continuous, even if you don't move the mouse.

It works like this in Warcraft for example. It even has a "bug":
If you open the menu with hotkey while scrolling with the mouse, and save the game, after it saved it, the screen will scroll, regardless where the mouse is, it only stops, when you move the mouse (so it's position updates in the game loop, while it didn't update in menu mode).

I hope that made sense.

Sorry, I was wrong, RTS games only scroll when the mouse is at the edge, so the first code in my previous post is to be used. Or if you have windowed mode, you could use long buttons too at the edges. If you push, you scroll, maybe it's a better solution, because it wont begin scrolling, if you move the mouse to the edge accidentally.

Or the other thing I mentioned, about the button thing:


So it scrolls, when you push the button (and keep it pushed) (red and green areas).

Of course, these buttons don't have to be windows GUI buttons, just draw something there to notify the user, that these are buttons to scroll.

Restricting to the window is meaningless, the user would want to click outside, because it's windowed. If full screen, no problem.

But there are tons of gurus in this forum, maybe there's a way to get mouse events even if it leaves the client area, because the window itself stays in focus. You could try to use GetCursorPos instead of message handling. It must return some value.

Don't wait until the mouse has left the viewport - at that point you're reacting too late. Instead, check if the cursor is in a region close to the sides, and move the camera accordingly. Keep in mind that if you keep those regions too small, you're going to frustrate players who aren't running your game full-screen.

The border-scrolling doesn't work too well for games that aren't full-screen, from a design perspective. Even in full-screen games, I often find that it gives me too little control over the camera. It's not precise enough imho.


Alternatives would be to use the arrow keys (good for keyboard + mouse control!) or to scroll the map while a mouse-button is held down and the mouse is moving (allows for fast and precise camera control). Whatever works best for your game. You'll probably want to provide multiple methods for different kind of players.