332992 - Top of text stripped off by tab bar during scrolling, may end up drawn on tab bar instead

Status: RESOLVED WONTFIX

(Core Graveyard :: GFX: Mac, defect)

Description

[Wayne Woods]

I hope this is the right component. It requires so many different things to generate this bug...

To reproduce:
1. Open at least two tabs. You can leave one blank.
1. Navigate to a page that has capital letters that are just the right size for a particular typeface. I can't say exactly what the formula for that is, but instead just load the test case, which I'll attach next.
3. Scroll down until roughly 1 px of text is cut off by the tab bar, but no more. Say, half the top bar of the "T" is gone, but not all of it.
5. Move the scroll bar back up again
6. If all goes to plan, the very tips of the letters are now missing, so they'll look flat on top. If not, adjust the text size and try again. I know this bug can occur or fail depending on text size.
7. If you scroll the text off and on, or highlight it, or change tabs, or do anything else to make the text redraw, it repairs.

But wait! Now for a tiny magic trick...

8. Repeat up to step 3, but *don't* move the scrollbar again. Leave it with the letters partially hidden by the tab bar.
9. Switch tabs, then switch back. Repeat this several times.
10. Eventually, voila! The missing tips of the letters are now drawn on the tab bar instead of on the page. You should notice little blots of black and colour on the bottom of the tab bar. They'll stick there until you move the tabs around, forcing them to redraw.

This bug works with a few typefaces I've tried, but the size of the text that is required seems to vary from font to font. For the example, I've used Verdana, because it's chunky and it's easy to see the effect. It requires a size of 17 px to facilitate the bug. Times New Roman requires 19 px and Courier 16 px. I've also added bold and colour to the test case in order to make it easier to see, but they're not required to generate the bug.

I originally discovered this bug on the Bugzilla Advanced Search page ( https://bugzilla.mozilla.org/query.cgi ) but I can't seem to get it to work today.

This bug occurs in both Firefox and SeaMonkey, and goes as far back as I was able to test (2003 builds). I'm surprised I can't find an exact duplicate, although there are a number of other bugs reported for Linux that deal with pixel rows being stripped from text (see especially bug 217336 comment 7 for a list). None are quite like this OS X one, though maybe I just haven't stumbled on the right dupe. I wonder if this also occurs in Linux...

Comment 1

[Wayne Woods]

Attachment: Test Case

A row of upper case letters in Verdana 17 px. Should show the bug.

Comment 2

[Wayne Woods]

Attachment: Screenshot of the bug, showing both decapitated text and the artefacts on the bottom of the tab bar

Comment 3

[Wayne Woods]

Also finally managed to reproduce with lower case letters, but the size requirement is different. For example, if you want to shave the top off a row of lower-case "o"s, Courier works at 21 px instead of the 16 px for upper case.

Comment 4

[Gordon P. Hemsley [:GPHemsley]]

This bug has been buried in the graveyard and has not been updated in over 5 years. It is probably safe to assume that it will never be fixed, so resolving as WONTFIX.

[Mass-change filter: graveyard-wontfix-2014-09-24]