I've seen this happen in some of the programs that come with ReactOS. I'll describe it for Wordpad and Notepad, but the symptoms are always the same.
First resize the window by dragging the bottom border up. If it's far enough up to be at the place where the ruler used to be, or even higher, it's no longer possible to resize the window by dragging the bottom border.
Indeed, if you drag the border far enough up to be halfway the drop-down boxes, and you click on the border at a spot where a drop-down arrow used to be, the appropriate list box will drop down, even though the border still looks like a border.
Drag the border far enough upwards (if you just tried the previous step, you'll need to resize it from the corners) that the status bar covers say half the area where normally the menu bar is displayed, the menu bar will cover the status bar. However, when you click on the bottom half of a menu, nothing will happen.
First resize the window by dragging the bottom border far enough upwards so that the status bar is half covered by the menu bar. As was the case in Wordpad, now only the top half of the menu bar can be used.
If you make the window so small that its client area is about three pixels smaller than the height of the status bar, it will no longer be possible to use the bottom border to resize the window.