857192 - [HiDPI] Image viewer has 1px of scroll

Status: RESOLVED FIXED

(Firefox :: General, defect)

Description

[Greg Edwards]

Attachment: imagepreviewbug.jpg

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:23.0) Gecko/20130402 Firefox/23.0
Build ID: 20130402030843

Steps to reproduce:

Use a HiDPI setup with a version of Firefox with Bug 844604 landed. (192dpi tested)
Navigate to an image.
Make sure the image zoom is set to fit the window.
Resize the window a little bit if necessary.


Actual results:

Sometimes there will be a scroll bar that provides one device pixel's worth of scroll.


Expected results:

There should never be scrolling when the image is fit to the window.

Comment 1

[Greg Edwards]

I should also add that this doesn't appear to affect Mac, probably because it forces the window size to logical pixels.
It seems the fit-to-window script is rounding the image size up to the next whole CSS pixel.

Comment 2

[Jonathan Kew [:jfkthame]]

I was able to reproduce a similar effect on OS X/Retina by setting devPixelsPerPx to 4, although that introduces various other UI issues, so nobody should be using such a setting.

On Windows, I can reproduce at 192dpi, but not at lower settings such as 120 or 144.

I believe the problem arises because the ImageDocument rounds its "available size" to integer CSS pixels, but in HiDPI situations this may lead to rounding up to a size that's a fraction of a CSS pixel (but a whole device pixel) larger than is in fact available. Testing a possible patch at the moment.

Comment 3

[Jonathan Kew [:jfkthame]]

Attachment: use float instead of integer CSS pixels to track available space for image

This fixes the issue, AFAICT. Tryserver build at https://tbpl.mozilla.org/?tree=Try&rev=1eb63c4f04d0.

Comment 4

[Boris Zbarsky [:bzbarsky]]

Comment on attachment 737540 [details] [diff] [review]
use float instead of integer CSS pixels to track available space for image

r=me

Comment 5

[Jonathan Kew [:jfkthame]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/a6ffd1baf90a

Comment 6

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/a6ffd1baf90a