After a quick look, this should probably be fixed upstream in “ActionFieldLayout aligned inline”. Not convinced that ellipsis is a good solution anywhere as it might result in an extra click. I'd defer to showing always full contents (title of this label) and rather oblige more scrolling to the user instead of clicking and wait time for loading corresponding page.
Showing here the example given in the description on iPhone 6/7/8 (375px, not even 320px our minimum requirement)

If we do ellipsis we hide too much IMHO.

With word-break:

Change 573620 had a related patch set uploaded (by VolkerE; owner: VolkerE):
[oojs/ui@master] FieldLayout: Break overlong words in labels of inline FieldLayouts

https://gerrit.wikimedia.org/r/573620

Change 573620 merged by jenkins-bot:
[oojs/ui@master] FieldLayout: Break overlong words in labels of inline FieldLayouts

https://gerrit.wikimedia.org/r/573620

Change 575115 had a related patch set uploaded (by Jforrester; owner: Jforrester):
[mediawiki/core@master] Update OOUI to v0.37.0

https://gerrit.wikimedia.org/r/575115

Change 575115 merged by jenkins-bot:
[mediawiki/core@master] Update OOUI to v0.37.0

https://gerrit.wikimedia.org/r/575115

@Volker_E my understanding is latest OOUI should be released but I'm still seeing overflow issues with username "My name is ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh"" in https://en.m.wikipedia.beta.wmflabs.org/w/index.php?title=AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH&action=historywrappppppppppppppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiing&action=history

Yo, that's not OOUI. That has also not been available in original task description, but added after my first take. 😞

was added prior to estimation and your first patch.

Can you look into this or do you want me to to pick this up? Possibly can revise https://gerrit.wikimedia.org/r/c/548537 tomorrow if you don't have bandwidth.

Change 577343 had a related patch set uploaded (by VolkerE; owner: VolkerE):
[mediawiki/core@master] Word-break overlong usernames in links

https://gerrit.wikimedia.org/r/577343

Change 577351 had a related patch set uploaded (by VolkerE; owner: VolkerE):
[mediawiki/skins/MinervaNeue@master] Word-break overlong usernames

https://gerrit.wikimedia.org/r/577351

Change 577343 merged by jenkins-bot:
[mediawiki/core@master] Word-break overlong usernames in links

https://gerrit.wikimedia.org/r/577343

On 375px:

This isn't the most beautiful, but an acceptable compromise for this edge case. Everything else would result in much more code (floating, width limiting etc.)

Change 577351 merged by jenkins-bot:
[mediawiki/skins/MinervaNeue@master] Word-break overlong usernames

https://gerrit.wikimedia.org/r/577351

Change 548537 abandoned by Jdlrobson:
POC: Long edit summary/titles/user breaking AMC special pages

Reason:
Superseded by https://gerrit.wikimedia.org/r/c/577343, https://gerrit.wikimedia.org/r/c/577351,https://gerrit.wikimedia.org/r/c/573620

https://gerrit.wikimedia.org/r/548537

Change 581039 had a related patch set uploaded (by Jdlrobson; owner: Jdlrobson):
[mediawiki/skins/MinervaNeue@master] Limit title on Special:Watchlist and Special:RecentChanges

https://gerrit.wikimedia.org/r/581039

Change 581039 merged by jenkins-bot:
[mediawiki/skins/MinervaNeue@master] Limit title on Special:Watchlist and Special:RecentChanges

https://gerrit.wikimedia.org/r/581039

Test Result

Status: ❌ FAIL
OS: macOS Catalina
Browser: Chrome
Device: MBP
Emulated Device: iPhoneX

Test Artifact(s):

QA Steps

Edit summaries

1. Look at edit summaries on https://en.m.wikipedia.beta.wmflabs.org/w/index.php?title=AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH&action=history wrappppppppppppppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiing history page and create a new edit with a long word (using one of the existing sumarries as examples. When editing the page will be added to your watchlist
2. ✅ AC1: Check the history page on a mobile device

1. ✅ AC2: Visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:Watchlist?hidecategorization=1&hideWikibase=1&limit=250&days=3&urlversion=2 and make sure the words are wrapping.

1. Check the same on

✅ AC3: Special:RecentChanges

Username
-Visit https://en.m.wikipedia.beta.wmflabs.org/w/index.php?title=AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH&action=history

❌ AC5: Confirm username is truncated with a ...
Long username still wraps.

Edit the page
❌ AC6: Visit Special:Watchlist and confirm the title is truncated with a ...

Test Result - HEWIKI

Status: ❌ FAIL
OS: macOS Catalina
Browser: Chrome
Device: MBP
Emulated Device: iPhoneX

Test Artifact(s):

QA Steps

Edit summaries

1. Look at edit summaries on https://he.m.wikipedia.org/w/index.php?title=%D7%94%D7%9E%D7%94%D7%93%D7%95%D7%A8%D7%94_%D7%94%D7%90%D7%97%D7%AA-%D7%A2%D7%A9%D7%A8%D7%94_%D7%A9%D7%9C_%D7%90%D7%A0%D7%A6%D7%99%D7%A7%D7%9C%D7%95%D7%A4%D7%93%D7%99%D7%94_%D7%91%D7%A8%D7%99%D7%98%D7%A0%D7%99%D7%A7%D7%94&action=history wrappppppppppppppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiing history page and create a new edit with a long word (using one of the existing sumarries as examples. Add page to your watchlist
2. ✅ AC1: Check the history page on a mobile device

1. ✅ AC2: Visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:Watchlist?hidecategorization=1&hideWikibase=1&limit=250&days=3&urlversion=2 and make sure the words are wrapping.

1. Check the same on

✅ AC3: Special:RecentChanges

Username
-Visit https://he.m.wikipedia.org/wiki/%D7%9E%D7%99%D7%95%D7%97%D7%93:%D7%94%D7%99%D7%A1%D7%98%D7%95%D7%A8%D7%99%D7%94/%D7%AA%D7%90%D7%95%D7%93%D7%95%D7%A8%D7%95%D7%A1_%D7%94%D7%A9%D7%A0%D7%99_(%D7%90%D7%A4%D7%99%D7%A4%D7%99%D7%95%D7%A8_%D7%A7%D7%95%D7%A4%D7%98%D7%99)

✅ AC5: Confirm username is truncated with a ...
Long username still wraps.

Edit the page
❌ AC6: Visit Special:Watchlist (https://he.m.wikipedia.org/wiki/%D7%9E%D7%99%D7%95%D7%97%D7%93:%D7%A2%D7%A8%D7%99%D7%9B%D7%AA_%D7%A8%D7%A9%D7%99%D7%9E%D7%AA_%D7%94%D7%9E%D7%A2%D7%A7%D7%91) and confirm the title is truncated with a ...

I can reproduce. Lets try a third time lucky...

Change 583772 had a related patch set uploaded (by Jdlrobson; owner: Jdlrobson):
[mediawiki/skins/MinervaNeue@master] Break title on watchlist

https://gerrit.wikimedia.org/r/583772

Change 583772 merged by jenkins-bot:
[mediawiki/skins/MinervaNeue@master] Break title on watchlist and scope non-hyphenated links

https://gerrit.wikimedia.org/r/583772

Change 583800 had a related patch set uploaded (by Jdlrobson; owner: Jdlrobson):
[mediawiki/skins/MinervaNeue@master] Also break word in .mw-contributions-title

https://gerrit.wikimedia.org/r/583800

OK that should do it.

Change 583800 merged by jenkins-bot:
[mediawiki/skins/MinervaNeue@master] Also break word in .mw-contributions-title

https://gerrit.wikimedia.org/r/583800

Test Result

Status: ❌ FAIL
OS: macOS Catalina
Browser: Chrome
Device: MBP
Emulated Device: iPhoneX

Test Artifact(s):

QA Steps

Edit summaries

1. Look at edit summaries on https://en.m.wikipedia.beta.wmflabs.org/w/index.php?title=AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH&action=history wrappppppppppppppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiing history page and create a new edit with a long word (using one of the existing sumarries as examples. When editing the page will be added to your watchlist
2. ✅ AC1: Check the history page on a mobile device

1. ✅ AC2: Visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:Watchlist?hidecategorization=1&hideWikibase=1&limit=250&days=3&urlversion=2 and make sure the words are wrapping.

1. Check the same on

✅ AC3: Special:RecentChanges

Username
-Visit https://en.m.wikipedia.beta.wmflabs.org/w/index.php?title=AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH&action=history

❌ AC5: Confirm username is truncated with a ...
Long username still wraps.

Edit the page
❌ AC6: Visit Special:Watchlist and confirm the title is truncated with a ...

Test Result - HEWIKI

Status: ❓ For Review
OS: macOS Catalina
Browser: Chrome
Device: MBP
Emulated Device: iPhoneX

Test Artifact(s):

QA Steps

Edit summaries

1. Look at edit summaries on https://he.m.wikipedia.org/w/index.php?title=%D7%94%D7%9E%D7%94%D7%93%D7%95%D7%A8%D7%94_%D7%94%D7%90%D7%97%D7%AA-%D7%A2%D7%A9%D7%A8%D7%94_%D7%A9%D7%9C_%D7%90%D7%A0%D7%A6%D7%99%D7%A7%D7%9C%D7%95%D7%A4%D7%93%D7%99%D7%94_%D7%91%D7%A8%D7%99%D7%98%D7%A0%D7%99%D7%A7%D7%94&action=history wrappppppppppppppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiing history page and create a new edit with a long word (using one of the existing sumarries as examples. Add page to your watchlist
2. ✅ AC1: Check the history page on a mobile device

1. ✅ AC2: Visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:Watchlist?hidecategorization=1&hideWikibase=1&limit=250&days=3&urlversion=2 and make sure the words are wrapping.

Note: I used Responsive as the device in Chrome Devtools device emulator in order to make the width more narrow to force a wrap.

1. Check the same on

✅ AC3: Special:RecentChanges

Username
-Visit https://he.m.wikipedia.org/wiki/%D7%9E%D7%99%D7%95%D7%97%D7%93:%D7%94%D7%99%D7%A1%D7%98%D7%95%D7%A8%D7%99%D7%94/%D7%AA%D7%90%D7%95%D7%93%D7%95%D7%A8%D7%95%D7%A1_%D7%94%D7%A9%D7%A0%D7%99_(%D7%90%D7%A4%D7%99%D7%A4%D7%99%D7%95%D7%A8_%D7%A7%D7%95%D7%A4%D7%98%D7%99)

❓ AC5: Confirm username is truncated with a ...
Long username still wraps.

Edit the page
❓ AC6: Visit Special:Watchlist (https://he.m.wikipedia.org/wiki/%D7%9E%D7%99%D7%95%D7%97%D7%93:%D7%A2%D7%A8%D7%99%D7%9B%D7%AA_%D7%A8%D7%A9%D7%99%D7%9E%D7%AA_%D7%94%D7%9E%D7%A2%D7%A7%D7%91) and confirm the title is truncated with a ...
List doesn't truncate

@Jdlrobson I'm wondering if I'm just not logged in with the right user or settings when I test these. If you look at AC5 in T237230#6023325 It behaved differently with a user with AMC on vs off.

In T237230#6023387, @Edtadros wrote:

@Jdlrobson I'm wondering if I'm just not logged in with the right user or settings when I test these. If you look at AC5 in T237230#6023325 It behaved differently with a user with AMC on vs off.

I think that's fine. The bug we're fixing was just AMC. I've asked @alexhollender for some clarification about the acceptance criteria. It's possible all these might be passes.
https://phabricator.wikimedia.org/T237230#6023287