User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.106 Safari/537.36 Steps to reproduce: 1. Load a page with a <select> that has very long answer options. (Example: http://jsbin.com/tesebidigu/edit?output) 2. Click on the <select>, move your mouse to the left of the <select> width, and then move down to try selecting an option. Actual results: The option isn't selected. The user must click the option directly under the <select>, or to the right of the <select>, for their click to register. Expected results: The user should be able to click anywhere in the <option> to select it.

It seems it's not reproducible on Windows. Is it specific to Mac OSX?

Two employees and one customer of my company have reproduced it on OS X. None of us have Windows machines.

Looks like it's an old issue. There was a behavior change in 19.0a1, in the following range: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=dac5700acf8b&tochange=cb573b9307e5 the dropdown menu was using only right side, and it's expanded to left side, but the expanded part doesn't accept click.

The box of the option element seems to be related. with the original testcase, the box of the option element doesn't cover left side, and you can click the lightblue area only. when I add the following style to the testcase: option { margin: -200px; } the box of the option is shifted to right side, and you can still click lightblue area only, but this expands the clickable area 200px to left. So, looks like there's mismatch between the displayed menu and actually clickable element behind it.

I'm guessing this is caused by bug 794038 from the regression range. The popup location actually moves. So maybe the "contrain to screen" logic is affecting the position, but the display list/hit testing don't take this into account.

This sounds a lot like bug 1230467. Is it limited to non-e10s?

(In reply to Jonathan Kew (:jfkthame) from comment #6) > This sounds a lot like bug 1230467. Is it limited to non-e10s? Yes.