I'm not sure what you're asking? You are correct that wikitext allows tabs (though they're pretty rare as it's hard to enter one). We intentionally show these with a special glyph (since 2012, I think) to make clear that they're not just one or more spaces. (In visual mode, we also do this for ineffective newlines which don't cause a paragraph break.) Do you think we should reverse this decision? It would make it harder to spot this and fix it.

Perhaps use gray color instead of the normal black to show that these arrows aren't arrows but glyphs for TABs?

This is good function. But if someone thinks not - maybe adding button like in external editors to show/hide markings for non-visible elements like tabs, newlines?

On a related point, when I copied and pasted a long list, all of the newlines were replaced by little carriage-return arrows like ↵ and all the content was run onto a single line. When it saved, Parsoid fixed everything (thank you, Parsing team!), but it was quite impossible to read, and I nearly discarded the changes to avoid corrupting the page.

In T153434#2991146, @Whatamidoing-WMF wrote:

On a related point, when I copied and pasted a long list, all of the newlines were replaced by little carriage-return arrows like ↵ and all the content was run onto a single line. When it saved, Parsoid fixed everything (thank you, Parsing team!), but it was quite impossible to read, and I nearly discarded the changes to avoid corrupting the page.

That's a separate issue, which is fixed now, I think it https://gerrit.wikimedia.org/r/c/VisualEditor/VisualEditor/+/335086 was the change that fixed it.

Change 550952 had a related patch set uploaded (by Esanders; owner: Esanders):
[VisualEditor/VisualEditor@master] Disable whitespace replacement in source mode

https://gerrit.wikimedia.org/r/550952

Change 550956 had a related patch set uploaded (by Esanders; owner: Esanders):
[mediawiki/extensions/CodeMirror@master] Don't replace whitespace characters in VE

https://gerrit.wikimedia.org/r/550956

Change 550952 merged by jenkins-bot:
[VisualEditor/VisualEditor@master] Disable whitespace replacement in source mode

https://gerrit.wikimedia.org/r/550952

As noted on the merged patch, according to https://caniuse.com/#feat=mdn-css_properties_tab-size Firefox still needs the prefixed property -moz-tab-size.

Change 552087 had a related patch set uploaded (by Jforrester; owner: Jforrester):
[mediawiki/extensions/VisualEditor@master] Update VE core submodule to master (c6b5e46fa)

https://gerrit.wikimedia.org/r/552087

Change 552087 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@master] Update VE core submodule to master (c6b5e46fa)

https://gerrit.wikimedia.org/r/552087

In T153434#5666303, @Schnark wrote:

As noted on the merged patch, according to https://caniuse.com/#feat=mdn-css_properties_tab-size Firefox still needs the prefixed property -moz-tab-size.

Done in https://gerrit.wikimedia.org/r/c/VisualEditor/VisualEditor/+/551210

Change 550956 merged by jenkins-bot:
[mediawiki/extensions/CodeMirror@master] Don't replace whitespace characters in VE

https://gerrit.wikimedia.org/r/550956

As a result of these patches, we will no longer display tabs as "➞" in source mode (NWE). They will still be displayed that way in visual mode though.

I'm not sure if the tab replacement is source mode was an intentional decision, or just something we inherited from VE when we made NWE and didn't think it through. But for reference, let me note that it caused problems in syntax highlighting before and we fixed it back then instead of removing it: T187694 T226478

Also – I was worried that using real tabs could cause syntax highlighting alignment problems when the tabs are used in the middle of a line, I was expecting that browsers would always render them as 4 spaces (or 8 spaces) instead of having tab stops. Luckily this doesn't seem to be the case, and it works fine.