327898 - Unnecessary wrapping of list item inside absolutely positioned div on reflow

Status: NEW

(Core :: Layout, defect)

Description

[Matt Brown]

User-Agent:       Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9a1) Gecko/20060219 Firefox/1.6a1
Build Identifier: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9a1) Gecko/20060219 Firefox/1.6a1

After resizing the window the spacer tab (rightmost tab in the menu in the test URL) is often wrapped down to the next line for no reason. Space is still allocated for the tab on the original line.

While resizing the window the tab is visibly jumping between two locations:
1) the correct location on the same line as the other tabs,
2) the wrapped position underneath them



Reproducible: Sometimes

Steps to Reproduce:
1. Open example URL (or attached test.html)
2. Resize browser window - actual size doesn't matter
3. Observe tab jumping between locations
4. Complete resize operation
5. Repeat steps 2-4 until the error condition with the wrapped tab occurs

Actual Results:  
The rightmost item in the list is wrapped below the other items even though space is allocated (and available) for it alongside them on the original line.

Expected Results:  
The tab show always remain on the same line alongside the other tabs unless the browser window is too narrow for that to occur.

I have attached the testcase and two screenshots, one showing the bug on the original site (www.wlug.org.nz) and one showing the bug in the testcase. 

This bug was initially discovered when viewing the site using firefox 1.5 and is also present in Seamonkey 1.0. In both Firefox 1.5 and Seamonkey 1.0 the bug is exhibited in the initial paint of the page (eg. no resize/reflow is needed to trigger it), but only when there is a vertical scrollbar present. The attached test2.html can be used to reproduce the bug for FF1.5 and Seamonkey1.0 if test.html cannot reproduce it.

Perhaps the latest nightly has fixed the bug for the on the initial paint case,  but not for reflows?

Comment 1

[Matt Brown]

Attachment: Minimal testcase to reproduce the bug in latest nightly

Comment 2

[Matt Brown]

Attachment: Minimal testcase to reproduce the bug in FF1.5/Seamonkey 1.0

Comment 3

[Matt Brown]

Attachment: Screenshot showing bug while viewing testcase

Comment 4

[Matt Brown]

Attachment: Screenshot showing bug on the original site

Comment 5

[Philip Withnall (unavailable)]

Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9a1) Gecko/20060220 Firefox/1.6a1 ID:0000000000
Confirmed. Both testcases initially render fine, but the spacer item jumps back and forth between a new line and where it's supposed to be when resizing the window.
When resizing the window, the spacer item will be on the bottom line, you can widen the window by one pixel and it'll jump back up. Widening the window by either 2 or 3 (varies) pixels more will make it jump back down again, and so on...

Comment 6

[philippe (part-time)]

Confirmed on OS X with
Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.9a1) Gecko/20060218 Firefox/1.6a1 ID:2006021805.

Comment 7

[Mats Palmgren (inactive)]

WFM, Firefox 2008040704 on Linux.
WFM, Firefox 2008040504 on MacOSX.