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Signal Processing answer, normal vs preview:

It's only toward the bottom of the post, yet it doesn't happen toward bottom of a much longer post. I confirmed it's also reproduced only with the offending bit, pasted below.

Reproduced in Firefox, Edge, Chrome, Windows 10, Windows 11. Reproduced on Cross Validated by pasting and submitting an edit temporarily.

<img src="https://i.sstatic.net/Gwa3Q.png" width="330">



`fft(x)[0]` is the plot summed. Note the primary signal here is obliterated by the window's excellent frequency resolution, and what remains is leakage. The image shows the `'constant'` case, with DC zeroed; its removal matters because `fft(win)` effectively behaves like `[1, -1, 1, -1]`, so every non-zero input makes a huge difference. Yet, removal of any bin adjacent to DC would have a nearly identical effect.

This explains the behavior in terms of DFT and resolves the apparent "paradox". I don't think it's possible, however, to do this for more than just one case at a time and prove dependencies on $\tau$ as we have. Sometimes the DFT has a DC, other times not, and adjacent bins also vary. It's what motivated my closed form solution.
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This is a bug. The backend rendering is not compliant with CommonMark. Given that it appears associated with raw HTML <img> tags, it's likely that there's something happening when Stack Exchange runs the process to strip disallowed HTML.

Workaround

Several of your blank lines have a single space character on them. While that should have no consequence, you can work around this issue by removing the space characters on the blank lines.

Reduced example

CommonMark

The test Markdown with spaces on some blank lines renders correctly on CommonMark's test site.

Markdown

A paragraph

<img src="https://i.sstatic.net/wRMuO.png">
 
paragraph #2, which should be separate, `has some code`.
  
<img src="https://i.sstatic.net/wRMuO.png">
 
paragraph #3, which should be separate, `has some code`.

paragraph #4, which should be separate, `has some code`.

Rendered by Stack Exchange (flawed)

A paragraph

paragraph #2, which should be separate, `has some code`. paragraph #3, which should be separate, `has some code`.

paragraph #4, which should be separate, has some code.

Using workaround of not having a space character on blank lines (renders correctly)

CommonMark

Rendering the workaround on CommonMark's test site: also renders correctly

Markdown

A paragraph

<img src="https://i.sstatic.net/wRMuO.png">

paragraph #2, which should be separate, `has some code`.

<img src="https://i.sstatic.net/wRMuO.png">

paragraph #3, which should be separate, `has some code`.

paragraph #4, which should be separate, `has some code`.

Rendered by Stack Exchange (correct)

A paragraph

paragraph #2, which should be separate, has some code.

paragraph #3, which should be separate, has some code.

paragraph #4, which should be separate, has some code.

Image credit

The image used is:

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  • I saw the space and wanted to test ridding of it at last minute, but lazied out. I think something similar happens with Sphinx docs in Python - what's with all these sophisticated programs botching up '\n'.join(l.strip() for l in txt.split('\n') if l.strip() == '')?? (rhetorical q)
    – OverLordGoldDragon
    Commented May 24, 2023 at 18:49

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