RI_MOUSE_BUTTON_1_DOWN

Undocumented in source but is binding to Windows. You might be able to learn more by searching the web for its name.
@system extern (Windows) nothrow @nogc
static if(_WIN32_WINNT >= 0x501)
enum RI_MOUSE_BUTTON_1_DOWN = RI_MOUSE_LEFT_BUTTON_DOWN;

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