1509159 - Vertical scroll bar generated by overflow-y: auto is not automatically disappearing

Status: NEW

(Core :: Layout, defect, P3)

Description

[pdelancie]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:63.0) Gecko/20100101 Firefox/63.0

Steps to reproduce:

I have a div with overflow-y: auto so that (in theory):
- the vertical scrollbar is present when the width of the div is too narrow for its full contents to be seen without scrolling.
- the vertical scrollbar is not present when the width of the div is wide enough for its full contents to be seen without scrolling.


Actual results:

If the browser window is wide enough when the page first loads so that the div is at its full width then, as expected, the vertical scroll bar is not needed and not shown. If you then narrow the browser window until the div starts to narrow, the vertical scrollbar appears (as expected). However, when you then widen the window again, thus widening the div again to its full width (in other words restoring the width to where it was when page first opened), in Firefox the vertical scroll bar does not go away as it should, though it does go away (as expected) in Chrome. 


Expected results:

Given the overflow-y setting of auto, the vertical scrollbar should only be present when the contents of the div can't be fully seen without scrolling. So when the width of the div has been narrowed to where the scrollbar appears and then widened again to where it's no longer needed (to see all of the content) it should go back to being hidden.

Comment 1

[Alice0775 White]

Can you please attach a minimal testcase or public url?

Comment 2

[pdelancie]

Thanks for the reply. Here's how to see it:
- Set your viewport (browser window) to at least 1220px wide by 300px high.
- Go to this URL: https://kb.kentik.com. The light blue div at upper right should have no scrollbars.
- Narrow the width to about 1200 or less. Vertical scrollbar should appear.
- Widen the viewport again. Vertical scrollbar should go away. It does in Chrome, but doesn't in FF. This is why I think there is a bug.

Thanks for looking into this!

Comment 3

[Alice0775 White]

Attachment: reduced html

Thanks, I can reproduce the issue on windows10 Nightly65.0a1.

Comment 4

[Alice0775 White]

regressed between Firefox2.0.0.22 and Firefox3.0.

Comment 5

[Alice0775 White]

Regression window:
Last good: BuildID=2008022806
First bad: BuildID=2008022906

Regressed by: Bug 405952