If I vertically decrease the xterm window size almost 50% percent of the window stays blank. Only after scrolling, moving the xterm window behind an other window and moving it to the front position again or increasing the xterm window size again vertically refreshes the window. If the window size is increase verically still one blank line remains.
Had similar problem with xterm-274. Switching workspaces in openbox also did not repaint the window in some occasions.
Bumping to xterm-275 seemed to fixed it for me.
275 is now in Portage, please test as per Comment #1
With x11-terms/xterm-276 I do not see this issue anymore.