try toggling the hardware acceleration setting in options/advanced / general

This is often responsible for odd artifacts in the user interface.

Thanks Matt,

I tried toggling the setting a few times though, unfortunately it did not work.

I am looking forward to get another advice.

Do these "shadows" show up in a screenshot? If so, could you share a screen shot of the problem with us?

What operating system are you using? I'm hearing grumbles about dissatisfaction with the use of gtk3 in recent versions of Thunderbird.

Thanks Zenos,

My PC with the problem is Windows7 64bit in my office. I also use with MacOS X in my home though, no effects appears on it.

Anyway, I show up a screenshot. I am sorry, but some of texts are shown in Japanese and I feel difficulty to recognize the problem on PNG format.

It is mysterious. When I zoom in on the screenshot, the shadow disappears.

It is possibly an optical illusion of the eye. My coworker cannot recognize the effect.

However, the shadow does not come out with my eye also, when the version is returned.

Is there any difference between previous version and current version?

I don't have an answers. But some thoughts…

Some Windows users were angry when Microsoft made Segoe the default font. It relies on "sub pixel dithering" to work well, and some users cannot work with this. They see coloured fringes.

I read that this is associated with Cleartype, and that Microsoft disabled this in their Office suite because it was affecting users' abilty to read.

Your screen is a grid of pixels. When letters are displayed, parts of those letters don't coincide exactly with pixel edges. So, in "full pixel dithering", the letter shape is approximated by shading adjacent pixels grey. "Sub pixel dithering" takes this idea further by using the individual red, green and blue elements of the pixel independently to do the "rounding off". For most people this works well; for a few, they see the separate colour at the edges. You may be one of this minority.

If this is what causes your fringes, then I don't know why you'd see it in Thunderbird but not anywhere else.

I do myself occasionally see coloured edges, usually on the right and left hand sides of letters with strong vertical elements, such as L and I. It is usually associated with particular fonts at particular sizes, most often with white-on-black text, and may be affected by another font adjustment known as hinting. So it's not surprisinfg that zooming changes it, since it changes the width of lines and where and how they fit into the pixel grid.

You may find that changing the font size, perhaps by only a small amount will help you.

Thanks a lot Zenos, I totally agree with your thought.

As I showed on previous comment with a screenshot, I almost all cut off window effects in order to get response better.

Now I tried to be returned to original, so the shadow was gone away.

I decide this problem is solved.

Thank you all very much.