(svn r27167) -Fix: [SDL, Windows] Right-mouse-button scrolling scrolled/jumped way to far, when OpenTTD lagged during mouse event processing.

src/gfx.cpp

@@ -1603,6 +1603,53 @@ void SetAnimatedMouseCursor(const AnimCursor *table)
 	SwitchAnimatedCursor();
 }
 
+/**
+ * Update cursor position on mouse movement.
+ * @param x New X position.
+ * @param y New Y position.
+ * @param queued True, if the OS queues mouse warps after pending mouse movement events.
+ *               False, if the warp applies instantaneous.
+ * @return true, if the OS cursor position should be warped back to this->pos.
+ */
+bool CursorVars::UpdateCursorPosition(int x, int y, bool queued_warp)
+{
+	/* Detecting relative mouse movement is somewhat tricky.
+	 *  - There may be multiple mouse move events in the video driver queue (esp. when OpenTTD lags a bit).
+	 *  - When we request warping the mouse position (return true), a mouse move event is appended at the end of the queue.
+	 *
+	 * So, when this->fix_at is active, we use the following strategy:
+	 *  - The first movement triggers the warp to reset the mouse position.
+	 *  - Subsequent events have to compute movement relative to the previous event.
+	 *  - The relative movement is finished, when we receive the event matching the warp.
+	 */
+
+	if (x == this->pos.x && y == this->pos.y) {
+		/* Warp finished. */
+		this->queued_warp = false;
+	}
+
+	this->delta.x = x - (this->queued_warp ? this->last_position.x : this->pos.x);
+	this->delta.y = y - (this->queued_warp ? this->last_position.y : this->pos.y);
+
+	this->last_position.x = x;
+	this->last_position.y = y;
+
+	bool need_warp = false;
+	if (this->fix_at) {
+		if (!this->queued_warp && (this->delta.x != 0 || this->delta.y != 0)) {
+			/* Trigger warp. */
+			this->queued_warp = queued_warp;
+			need_warp = true;
+		}
+	} else if (this->pos.x != x || this->pos.y != y) {
+		this->queued_warp = false; // Cancel warping, we are no longer confining the position.
+		this->dirty = true;
+		this->pos.x = x;
+		this->pos.y = y;
+	}
+	return need_warp;
+}
+
 bool ChangeResInGame(int width, int height)
 {
 	return (_screen.width == width && _screen.height == height) || VideoDriver::GetInstance()->ChangeResolution(width, height);