Bug: Mouseover translation hints "flashing"
In several translation documents, putting my cursor over some words in source text (left column) produces a "flashing" popup effect. The mouseover translation-hints box pops up and disappears again 4-5 times a second. This effect happens only when I've first activated the text entry box to the right of the source text, to enter a translation.
You can see this on the page I'm currently translating, '¿Cómo puedo enriquecer mi yogur?' (http://duolingo.com/#/translation/9560e98b1bf6041412c473c4b9c66a8a). Click in the text entry box to the right of the first paragraph, which 'activates' the box, inserts extra horizontal and vertical space in the source text, and puts each word in a 'button,' Mouseover the word 'desnatada' to see the flashing popup. This effect may occur only in my current browser, Firefox v18.0.2.
This bug does two things. First, I can only read the contents of the popup box by putting my face about six inches from the screen, and fixing my gaze on the popup box (it always appears in the same place) until my eye has taken it in enough times that I can read it. Second, the text on either side of the problem word "jumps" forward and back, as the flashing popup changes the length of the line and the line breaks change back and forth. The affected text is nearly impossible to read while it's jumping around. The flashing popup box and jumping text stops as soon as I move the mouse cursor off the problem word.
I only see this problem in text to the left of an active text entry box, when additional horizontal space has been inserted between lines, and extra vertical space between words, and each word is displayed in its own button. I only see it happen with words at the beginning of lines. Changing the width of the browser window has no effect.
I see this problem while using Firefox v18.0.2, my primary browser. I don't see it when I use Internet Explorer; my current version is v9.0.8112.16421 64-bit edition. (However, I've only checked a problem page in IE twice. The problem may occur there too sometimes, but I just haven't seen it yet.) I can't say whether the problem behavior is caused by the browser per se.
This "flashing mouseover" bug isn't fatal. I can read the contents of the popup box if I look at it long enough, then move my mouse cursor off the source word. But the flashing popup box and jumping text are very distracting. I see this bug on about one of every 8-10 pages.
I'm encountering this bug in the Swedish course, 2 years later. (One sentence triggering it, in FF Developer edition 41.0a2 on OS X Yosemite, is "Båda hans föräldrar är professorer.")
As mentioned in the original post here, it's not a killer bug, but it is a minor nuisance.