That is really difficult to compute... background and foregrounds colors not the same, hidden z-index, not truncated by fragments, overflow, ellipsis. 

You basically need to raster the text and then OCR it because fonts could map the glyphs to things that don't match text.

What website is bothering you with tooltips? Perhaps a site is being overly zealous with them.

Perhaps I should have narrowed the scope a bit.
I am working on a project where a request has come up (and implemented in JavaScript) where the tooltip should only show up if the text is not truncated (using ellipsis).
The way to do it is check whether scrollWidth === offsetWidth (or something similar) and if not, add title according to the textContent, otherwise, remove the title (if it happened to have been there before).
This seems like a reasonable request - why would the tooltip show up if the text is completely shown, right?
And you cannot know that in advance (text-overflow: ellipsis exists for a reason), so you have to detect whether it did not fit in runtime, which seems silly, because there is no point in such a tooltip in the first place.
Maybe adding a <element showoverflowtooltip> attribute (woo hoo, bikeshed) to the web platform could solve that, but it seems like a default setting should be applied, at least for potentially ellipsized content.

So to narrow it down - show tooltip if overflow: ellipsis/clip actually added an ellipsis/clipped the text. Is that more reasonable to implement?

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

As per comment# 0 from the reporter, issue seems to be a Feature request, hence marking it as Untriaged.

Thanks!

Trying different component -- this is not an accessibility bug.