I see the behaviour I described on both my Surface Studio desktop with NVIDIA GTX980M graphics and Windows 10, and on my Surface Laptop 4 with Intel Iris Xe graphics and Windows 11. On both Windows and graphics drivers are up to date. On both the IDE width is restricted to the width of the primary display.
Windows Forms apps, like Basic4ppc don't appear to suffer this limitation, nor does a WPF 3D map viewer that I wrote, nor do other apps I have tried. On my two systems it does seem to be B4X IDE specific.