Also note that the forum parts are underlapping the sidebar.

Can you tell me what browser you are using before I pass this onto our Programmers?

It's not browser specific, it's related to the media query breakpoints you have set in your CSS.

You can recreate in any browser simply by adjusting the window size or the screen resolution.

I get that in safari when browser window 940 px.

didn't notice before, as browser window usually 1280 px.

This is in Firefox. 

I bought a 1920x1080 monitor, ironically to make Poser work better... but I've found that there's no way to make fonts large enough for my ancient eyes to see properly at the default resolution.  So I finally set it down to 1600 x 900, which tends to squeeze things in strange ways but makes letters nicely visible.

As Fenier says, this isn't really browser-specific.  But in the bizarre world of RWD all exceptions have meta-exceptions and all meta-exceptions have meta-meta-exceptions, and so ad infinitum.

they may be able to help with poser font sizes, e.g. in UI/xml pref files.  admittedly not user-friendly, but they'll know more in poserforum.

This looks like its the result of using something like bootstrap (which I see in the source, is in fact used here).

Basically you have CSS designed to detect a mobile device and adjust everything when found. It does so using the window size. So adjusting your window can cause the screen to go into mobile mode - where buttons get moved and things go single column, etc...

But if mixed with other plugins that use different methods for mobile detection, it can get really weird when not actually on a mobile device - and you get things like ads displaying over places that have moved because they think they're mobile.

Bootstrap is great - I love that plugin and use it myself at my work. It really smoothes out a lot of CSS issues. But it also assumes people are on more modern displays and that the chances of them zooming the display, using a small resolution, or even using a window that is much smaller than the rest of the screen... are small.

Quote - make fonts large enough for my ancient eyes to see properly at the default resolution.  

Get glasses.

I had the optometrist tell me the approximate diopter for each eye then bought a bunch of cheap reading glasses at the drug store and swapped the lenses (.025 in my right and 1.25 left).  I have the expensive pair (bought through the optometry shop) for computer work and keep my cheapo glasses all over the house.  I have several in the garage for working on stuff and even tied a pair to the inside of my welding googles.