Comment 2 for bug 419523

Openbve upstream response is that the first returned mouse-result via SDL after changing between fullscreen and windowed mode (also via SDL) is bogus:

"Summary
When mouse grab is activated and the user toggles to or from fullscreen mode, the viewing angle might change abruptly to a nonsense angle.

Background
SDL reports nonsense values for the relative mouse position after switching to or from fullscreen mode, likely due to the fact that the screen dimension just changed. This will be circumvented by ignoring the first round of values that are reported by SDL for the relative mouse position after toggling fullscreen mode."