I'm afraid it may be a while until we see a fix in chromium as this bug has been open since October 2013 and it's currently unassigned. It's unfortunate that Blink is the last engine to sport this bug. The CKEditor comment that believes that the bug will be resolved soon in Blink is over 30 months old!
I know that this situation is very specific, using autocomplete on a hidden contenteditable, but it may be a possible scenario for single page applications where parts of the dom may be hidden until needed.
I did not think of inheritance. If I understand correctly the originally proposed code would fail for all browsers in cases where an element has inherted the contenteditable property?
May I suggest a possible solution that will not cover all the chrome bug cases but should provide some relief for chrome users and would still work on other browsers at the expense of an another check:
this.isMultiLine = isTextarea ||
!isInput && (this.element.prop( "isContentEditable" ) || this.element.prop( "contentEditable" ) === "true");
This would fix the bug in chromium browser in instances where the element on which the autocomplete is applied has the contenteditable property set directly.
The fix seems ugly though and would add code that exists only because of a specific browser bug. Also, once the chromium issue is resolved the fix should be reverted, so this would add a bit of debt to the codebase. And I agree that this may not be a widespread problem.