What version are you running?
Review Board 3.0.7
What's the URL of the page containing the problem?
confidential
What steps will reproduce the problem?
No conclusive steps to trigger this issue (although it seems to be happening consistently on the same file in the same review (no matter the revision as long as it includes that file))
What is the expected output? What do you see instead?
The expected output is the "before" diff on the left and the "after" diff on the right half of the screen. Instead, the left part is displayed normally the middle buttons for viewing nearby code is pushed to the end of the page (to the right) and is half beyond the end of the screen (as the whole right ("after" diff) colum is).
What operating system are you using? What browser?
OS: Microsoft Windows 10 Enterprise (64 bit) Version 10.0.18363 Build 18363

Browser: 

Google Chrome Version 79.0.3945.130 (Official Build) (64-bit) - the issue persists

the latest Opera - the issue persists

the latest Mozilla Firefox and - the issue is not seen

the latest Microsoft Edge - issue is not seen

the latest Microsoft Internet Explorer - issue is not seen
On my Ubuntu 18.04 box the issue is seen with Chromium and it is not seen with Firefox, so I believe it is browser dependent and as far as I can tell, it's seen on Chrome/Chromium and Opera consistently.
A colleague on MacOS first noted the issue, but I'm not sure what browser he was using.

Please provide any additional information below.
The right column with the "after" diff can be seen in the html, but it falls off the screen (this happened only for 1 of 135 files in the review)
Here is a snippet from the review html which shows that the second part is present (although not visible as it's outside the screen):
<tr line="112" class="first last">
<th>

 <a name="12.6" class="chunk-anchor"></a>

112</th>

   <td class="l">

<pre>      CPPUNIT_ASSERT(host != <span class="hl">NULL</span>);</pre>

</td>
<th>112</th>

   <td class="r">
<pre>      CPPUNIT_ASSERT(host != <span class="hl">nullptr</span>);</pre>

</td>

  </tr>