What happens is when the pop ups pop up some of them are bigger ( taller ) than others
Right now the Ebay one just came in & went out, and the window did not change.
The following add was thinner and the whole reply pane moved up the screen about 1/4"
Then there was no add and the pane moved another 1/4" up the page .
The top bar add always fits between the black bar at the top of the window & the bottom off the grass in the headder.
So that is fine
The second strip add rarely sits neatly in the blank space between the thread navigation bar and the top green bar
It looks like the text ( Reply to Thread) is justified to the bottom of a text box with a blank line above that goes completely across the page
So then the 2nd strip add loads it expands the window, we might be using redraw in different sense , so I will just call it expand as the screen does not go blank, it just moves down.
So if the 2nd strip add is the same size as the top strip add the window expands around 1/2".
I run Firefox full screen so the movement is just annoying but if the window is narrower it can cause the text input to drop off my screen.
Now this could be because you are using functions not available on my older browser version.
Then every now & then the 2nd strip add draws bigger then the top strip add and this pushes the region of the screen I am using even further down the page.
How much the screen bounces around depends upon haw fast the adds cycle and their sizes.
So it looks like some formatting is needed for the space where the second strip add loads.
A Mitsubishi add has just appeared, it was dynamic and while the image was changing it hyjacked my processor so what I was typing did not appear till the add vanished.
Most of the time there are no adds at all but I suppose that is a regional thing as I only get regional adds for things I have bought or places I have been.
Then some times the top two adds are identical but the lower add draws larger than the top add which pushes the screen even further down.
When typing a reply it is not too bad apart from your eyes being forced to bounce up & down but if you are reading a post near the bottom of the screen then the entire section you are reading will vanish off the screen as it gets pushed lower than the viewing area.
I took a couple of images to show you this.
There is a problem with a lot of web developers & designers who only run the lattest versions of everything on the most modern equipment and don't bother to check their fancy dancy handiwork will actually work properly on older machines and older browsers.
Took a couple of screen shots to illustrate
No add
Normal size add