That is, in particular, needed for Disgaea 5 complete to render movies with 'MovieType=1' settings (that settings is auto selected initially by the game based on GPU). Movie render window is initially a top level window, then the game reparents movie rendering window to the main window with SetParent() after 3d rendering to game window was already initialized earlier. The offscreen state for the window being rendered to (child in this case) is updated during presentation, but the parent does not (the same issue would be for the things going other way around, presenting to parent window when the child is never presented after reparenting).
This part is not enough to fix these cases though since the recent commit 786d9d1685ac220081b10cc779d4d331ddd2fc52 . When parent is not presented but the child is, the child image should appear in the window regardless of the fact that the parent has no WS_CLIPCHILDREN attribute. I know that commit is fixing regression Bug 57503, but due to these reasons the commit doesn't look right. And it is not clear to me from bug description what was actually wrong with "World in conflict" rendering when parent window offscreening took place: that alone is not supposed to be breaking the rendering (and if it would that would probably be a blocker for implementing child windows the way they are implemented?), I suspect there might be a bug elsewhere (just e. g., some other missing case of synchronizing window config state which ends up in child window not being properly offscreened in that case).