Looking into this a little more, there is code in the position reset routine that checks if the mIRC main window is 10 pixels less than the leftmost border of the screen. If it is, mIRC's leftmost position is set at zero. This was implemented a long time ago to prevent mIRC disappearing offscreen in some situations.
Windows 10 uses invisible borders for main application windows of about 8 pixels. If you are trying to position mIRC left/right flush with the screen, it is possible that this is the cause of the issue. When mIRC resets its position to zero, it looks like it is not flush with the left of the screen, even though it actually is at position zero - the invisible borders make it look like it isn't.
The Windows 10 invisible borders are a tricky issue to resolve - they are not backwards compatible with how most applications set window size/position and most applications experience issues with them, eg. see
here,
here,
here,
here, and
here. In the last link, someone found a solution, however applying it is difficult - it would require changing hundreds of places in the code that set/get window position/size and even then it is not clear if this would resolve the issue or introduce new ones in other contexts.
I will see if I can find a way of calculating the invisible border width and, in this specific context, resetting the position to be flush with the border, however I cannot guarantee that I will be able to find a working solution.
