Works fine for me in Chrome & Firefox.

I can reproduce it consistently in Safari 10. (In step 2, I'm triple-clicking the title. Perhaps that matters?)

I can reproduce in Firefox, both with h1s from other wiki pages, and with the h4 from https://jquery.com/upgrade-guide/3.0/#deprecated-document-ready-handlers-other-than-jquery-function. The headline is inserted as HTML, but not recognized by the editor, neither does it enable the "Save" button, nor can it be undone (if it wasn't the first change, "undo" will undo the change before pasting). Ctrl-Shift-V works fine.

@Schnark Tested the example you gave in Firefox, I get 'link Deprecated: document-ready handlers other than <code>jQuery(function)</code>' in the editor. Do you get an errors in the console?

I see an exception in Safari. Worryingly it isn't limited to NWE - bumping priority.

All paste broken in Safari - raising again.

Change 333359 had a related patch set uploaded (by Esanders):
Check dataTransfer.items exists before iterating

https://gerrit.wikimedia.org/r/333359

Change 333359 merged by jenkins-bot:
Check dataTransfer.items exists before iterating

https://gerrit.wikimedia.org/r/333359

Change 334089 had a related patch set uploaded (by Jforrester):
Update VE core submodule to master (f1ed206)

https://gerrit.wikimedia.org/r/334089

Change 334089 merged by jenkins-bot:
Update VE core submodule to master (f1ed206)

https://gerrit.wikimedia.org/r/334089