Upgrade plans for MathJax v3/v4

In preview, display math $$ ... $$ is incorrectly rendered as ordinary in-line math $ ... $.

E.g., when editing this answer, the preview looks like this:

It shows up correctly in the final post.

Strangely enough, editing this post shows an opposite behaviour: $$ ... $$ displays properly, but normal paragraph breaks are not rendered. Also, Markdown formatting including *...* and **...** is not rendered:

Again, it shows up correctly in the final post, and in v2.

MathJax v4 was released yesterday. It includes line breaking, a default font with much greater character coverage, and a choice from among 11 different fonts, the begingroup extension that was missing from v3 (with extra features for isolating posts specifically for sites like SE), and a number of other important new features and improvements. See the release notes for details, and the updated documentation, particularly the What's New in MathJax v4.

I believe the nearly all the issues mentioned here are either addressed in v4, or can be addressed by using the proper configuration.

In many of my answers on Math SE and MathOverflow, I use the \label MathJax command with equations, e.g., \label{eq1A}, so I can later reference them using the \eqref command, e.g., \eqref{eq1A}. This, however, doesn't work properly with the new version's editor, with it complaining about each of the labels being duplicated, even though one of them are actually being used.

For example, with my answer on Math SE, when the option is turned off and I edit the answer, the top part of what I see is:

However, when I turn on the MathJax v3 beta option, the top part of this is now:

These yellow multiple definition error messages continue further for every equation where I used \label. Note this problem also occurs while I'm initially creating a new answer. Nonetheless, apart from requiring a refresh if I save my edits, the final display does otherwise work properly (including if I'm only just displaying these answers).

It seems v3 has some issues with bounding box of some expressions with exponents. Here are two examples I found now - notice the exponents being cut out:

source: 0\leq(x(x-a)(x-b))^{2^n}=-1.

(from https://math.stackexchange.com/a/2647669/290240 )

v3:

v2:

source: x^2+x-\zeta_{2^{n+1}} \equiv x^2+x-1 \in O_K/(\pi)[x]\cong \Bbb{Z/2Z}[x]

(from https://math.stackexchange.com/a/3340575/290240 )

v3:

v2:

I'm sure there are better examples around. I think I saw v2 doing this too but it was very rare, in current v3 beta I see it much more often.

Edit: I just noticed that the second example shows how v2 and v3 render the number two at the end differently. But I guess v3 is correct in this case, since \Bbb{} applies to the whole Z/2Z.

I added a comment using the MathJax \color command to make several parts of a couple of equations be red. To better show the text that I used and the results, I repeated this using an answer on Math meta's "Formatting Sandbox" post. Here's what it looks like with the option turned off while editing:

After saving this, both the post, as well as the comment, appear correctly. However, when editing that post with the MathJax v3 beta option turned on, it instead appears as:

Notice how the first equation now has all of it being red (as shown in the first blue oval) after the first usage of \color, as opposed to just the intended 3 separate smaller parts. Also, in the second equation on the next line, the last 3 terms (in the second blue oval) are red, instead of just the the last term of the LHS of the equation.

Unfortunately, this problem persists when I view this post, as well as the original comment.

Note that wizzwizz's comment below states:

This behaviour is documented; there's a fix available, but I think this should only be applied to old posts, if possible.

Fortunately, one quite simple way to have the color handling work properly in both MathJax v2 and v3, as it's now without any potential fixes or other changes being implemented, is to use brace brackets around each of the color commands and text, so they form their own groups. For example, we can change \color{red}{2abc} in the first equation above to {\color{red}{2abc}}. More details, including the updated MathJax text, is in the Math meta answer I mentioned earlier.

This issue may potentially cause a syntax error to be displayed instead of the equation. Using MathJax v2, the last part of this answer is:

The issue is with the red '+' superscript being used. In MathJax v3 beta, this causes the equation display to become:

Note that the edit preview shows the same behaviors as in both the corresponding v2 and v3 beta displays. Fortunately, my earlier suggestion of using brace brackets around the entire part also solves the problem here for both v2 and v3 beta, i.e., in the MathJax for the equation of

$$\lim_{x\to0}\sqrt{|x|}\ln|x|=
\lim_{x\to0^\color{red}{+}}x^{\frac12}\ln x=0.$$

I changed in the middle part of the second line the \color{red}{+} text to {\color{red}{+}}.

With MathJax v2 it was possible to copy/paste a line containing MathJax into a plain Text editor. Of course this gives only a simplified representation.

For example, this is what I get when copying the title of this Math SE question on a Mac into a TextEdit window (raw format), or into BBEdit:

Determine a unit vector which is perpendicular to 2𝑖+𝑚𝑗−3𝑘 and parallel to 2𝑖+𝑗−𝑘.

With MathJax v3 I get only

Determine a unit vector which is perpendicular to and parallel to .

so apparently all math content is omitted from the copy.

Certain special characters are not currently being handled appropriately with MathJax v3 beta. For example, the top part of Sea ships and their space-time equations, in MathJax v2, appears as:

However, with MathJax v3 beta, it now shows as:

i.e., "Math input error" is displayed instead for both equations. The editor display of that part of the question is:

What's interesting is that the problematic character displays as a space followed by a question mark enclosed in a square. However, this display isn't the same here, e.g., with

x ⃗
⃗

Instead, as you can see, it appears to be a small square (at least for me, with this being similar to how unsupported characters with the current font are displayed), and a small arrow when shown by itself. However, on MathJax supported sites like the Math meta one, the square box is displayed as an arrow as well! This is a Unicode character that causes a backspace and overlay, which is how the "x" with an arrow overtop of it is shown in the top MathJax v2 output. Note that one may get a very similar result using MathJax with $\vec x$ instead.

There are also other special characters that work in MathJax v2 but result in the yellow "Math input error" message with MathJax v3 beta. For example, in Revision 4 of Find the smallest constant $C$ with given condition., the OP used the Euro symbol character. Unlike the other character mentioned above, this one appears to always be displayed properly, e.g.,

$€$
€

I'm unsure about why this problem is occurring with certain characters in MathJax v3 beta. I checked, but didn't find, this issue in the Upgrading from v2 to v3 - MathJax 3.2 documentation Web page. My suspicion is that MathJax v3 may be trying to interpret them as being some sort of unrecognized special MathJax characters.

In v2, the quote characters “...” (U+201C and U+201D) display as themselves just like most other Unicode characters:

In v3, they are engorged into huge double stroke characters:

(The screenshots are from this MathOverflow question.)

Another preview bug: < followed by a letter inside a MathJax expression renders the rest of the paragraph invisible (as it is presumably misparsed as a start of an invalid HTML tag).

It works correctly in the final post, and it worked even in preview in v2.

With MathJax V3 enabled, How can i Find the Area of a Traingle Formed inside a Triangle? has 5 initial comments, each with their date/time display being a link. However, clicking on "Show 5 more comments" then causes the original 5 comments, as well as the next 5 comments which are shown, to not have any of those date/time values being links any more. With Microsoft Edge, version 134.0.3124.51, on Windows 10, the Developer Tools display shows:

The final part, i.e., "Uncaught Error: MathJax retry", appears just after I click to show more comments. Thus, this seems to be what's causing the problem, or at least it's closely related. However, using MathJax V2 instead (e.g., while logged in, with an Edge private window, or logged out in Google Chrome), does always work properly.

Note that this problem doesn't occur every time more comments are shown, e.g., it works properly in Can $i^a$, for real $a$, be expressed in terms of $\cos$ and $\sin$ functions$?$. Instead, I believe that it's due to the sixth comment, i.e., the first new one shown. With V2, this appears to display properly as

but with V3 it instead appears as

In this case, none of the comment's MathJax is rendered. In particular, I suspect the problem is caused by the use of the \verb command. Nonetheless, as suggested in Emil's comment, just like with Are there any problems in mathematics where it is important to know if an entire function has infinitely many zeros? on MathOverflow, they are rendered correctly after copying and pasting those comments into an answer box. Also, most interestingly, all of the earlier comments now also appear properly! My suspicion is that the comments don't normally properly handle certain special functionality, with it being the use of \color with the MO question, as I indicate in my other answer, but that having them being in an answer box causes something to be loaded and/or updated so that it's then also handled properly in the comments.

This Math SE answer has the MathJax code of

$$y=x^2-(x-2)^2=4x-4\\
y=x^2-(x-8)^2=16x-64$$

With MathJax v2, this appears as expected, i.e., in 2 lines,

However, with MathJax v3, the equations appear together instead, one immediately after the other, all in 1 line,

As indicated in Martin R's comment, the page The line break(\) is not work · Issue #2312 · mathjax/MathJax · GitHub has an Oct. 2, 2019 comment by dpvc of

Linebreaks are not yet implemented in version 3. This is one of the significant features from version 2 that has not yet been ported to version 3.

That same user stated on June 22, 2020

... So line breaking and other features will most likely have to wait until next year.

There doesn't appear to be anything more about support for this in v3, but dpvc wrote on July 25, 2023 that

... line breaking support is in v4.0-alpha released last year, and in the 4.0-beta released last week. The official v4.0 is planned for the end of August.

but then on Oct. 27, 2024

... We are dealing with some issues with the changes made in the assistive technology support that still need to be worked out, and it is taking longer than anticipated. there will need to be one more beta release before the official v4. The betas are in use in production sites, so if the need is urgent, you could consider that. The current beta.7 is pretty close to the final version, and the beta.4 release predates the problems that we are working on, if you want to try those out.

Rather than waiting until v4, note there are several workarounds mentioned in that page.