I looks like there is some invisible break point there. I can't determine why the menu is horizontal, then flips to vertical at 1001 px.

Can you share a link?

I figured it out. There is a bug in RSD dealing with the slider in a EM based layout, when the users (in Windows) is scaling the display. When the user picks a breakpoint in RSD's window, the breakpoint is invisibly set higher, by the scaling factor setting in Windows, but breakpoint remains displayed where the user set it at. When previewing in a browser, it behaves as normal.

Example. Browser text set to CSS's 16 base size font (EM=1), EM based layout, Windows Scaling set to 125%, breakpoint set at 450px (28.125em). Design then actually breaks at 562px when moving slider, but no breakpoint there. Previewing in a browser, will break at 450px (28.125em). Clicking on breakpoint returns the slider, but have to slide to right 25% of the px setting for it to break.

Its repeating for me with a blank EM theme.

Also, in many of the EM themes, I can not increase the text size in the Design tab in Typography with mouse clicks. And can only try to click one size at a time. Clicking does nothing except sort of "flash" that section where it disappears and reappears very fast.. Must use the keyboard, or type in size. This is when windows is scaled up. Not all projects are like that though. Flux definitely is. Even going back to a scale factor or 100%, it can still happen, but the size can be increased once click at a time, but the screen is still "flashing" off and on.

The other options to increase/decrease line size, indent, word and character spacing work fine. Its just the text size change box that is goofy.