Bug: new thread's preview pages are wider than window size

There is a bug in LQ that I have seen since long ago, but it does not always happen (there is a catch), and only today I have isolated the problem.

I will write details here that may help you repeat the problem. The repeating steps may not be (and probably are not) limited to my test setup here. Other browsers, versions and distros may be different - but if your results are different, do not say the bug does not exist. It does.

OS: Debian 9
Browser: Firefox ESR 52.2.0 (64 bit)

Browser settings:

    - completely disable javascript
      ( about:config > javascript.enabled > false )

    - force the use of different colors for all
      pages:
         about:preferences#content >
         click the "colors" button >
         choose:
             1] almost pure white for text;
             2] any dark color for background;
             3] light colors for links;
             4] in the menu named similar to 
                "substitute pages colors with the 
                above", choose the option "always"
             5] instantly, when you click "OK" after 
                doing 4], the page colors in all tabs 
                change

Steps to do in a LQ profile:

1) login with your account, if you are not already logged in;
(clicking above here will not work, copy and paste this to the LQ domain:
/questions/login.php
)

2) open any thread with a post *you* have made (preferably that does not have [code] tags in the same page as your post); for my example, I am using
this thread
(
won't work, copy and paste:
/questions/showthread.php?p=5794092#post5793896
)

3) make the Firefox window, where you have your post shown, have a fairly small width (if your screen is too wide); a width close to 1000px should be good (and precision with that width is not needed);

4) zoom in that page until the thread fonts are fairly large; I have a zoom of 100% in this screenshot and of 160% in this screenshot (same window width for both, and a "ctrl+l enter" to make the scroll stay in the same post start);

note: in these 2 images, the post's first paragraph adapts to the new practical width its area gets with each zoom; what was in less than 2 lines in 100% has become 4 lines in 160% with my zoomed in font size; the LQ right menu also adapted to the new practical width, there are a few items that occupy two lines in 160% zoom;

5) keep the zoom at 160% from here on;

6) click to edit your post (do not worry, we will *not* make any permanent change);

7) make your post have a very long first paragraph (like my example post, with a 1st paragraph that occupy 4 lines @ z 160%), and make a second paragraph be as long (be sure to separate each paragraph with an empty line: two enters!), and make several more short paragraphs after these 2 long ones (1 word is enough for them, the idea is just to make the edited post vertically big);

8) click the "preview post" button;

9) scroll the preview page to the start of the post you are editing, now you should be seeing something similar to this other screenshot (I did the described previous steps);

note: the 4 line paragraph is only 3 lines now, this is not the problem, it is just because the date and poster info are not being shown; the important things here are: the text is being adapted to the fairly small and limited width we imposed; keep this preview tab open (we will copy the post text as in the preview);

10) open a new tab for LQ, choose any forum, click to create a new thread; copy all the test text we used to create the good preview we have seen in 9) and use it as the body for our new post (put any subject); you will be seeing something that is (surprisingly!?) larger than our window width!

So this is the bug: when creating new threads, the previews are (somehow, I did not care to search the reason) not limited to our window width. This makes it much harder to do as I prefer when creating new thread: I preview the post several times before starting the thread to avoid editing the post as the first readers see it (sometimes are big edits).

I started to create a new thread with the copied text, and I needed 2 shots to see the full width of the new thread preview:

left part of preview

right part of preview

Probably will not be hard to get fixed. Do you agree with that?