Page display is not accurately reflecting the growth/shrinkage at 290% and at 280% zoom levels




a year ago
a year ago


(Reporter: JuliaC, Unassigned)



Firefox Tracking Flags

(firefox49 affected, firefox50 affected, firefox51 affected, firefox52 affected)




a year ago
[Affected versions]:
- 51.0a2 (2016-10-17)
- 52.0a1 (2016-10-17)
- 50.0b8 build1 (20161017130949)
- 49.0.1 build3 (20160922113459)

[Affected platforms]:
- Windows 10 x64
- Ubuntu 16.04 x64
- Mac 10.11.6

[Steps to reproduce]:
1. Launch Firefox
2. Go to a random site (e.g.
3. Zoom in the chosen page by pressing [Ctrl] button and spinning the mouse wheel upward until it reaches 300% zoom level
-  inspect the page zoom level
4. Zoom out the chosen page by pressing [Ctrl] button and spinning the mouse wheel downward until it reaches 100% zoom level
- inspect the page zoom level

[Expected result]:
- the Zoom Indicator as well as the page display are accurately reflecting the page zoom growth at each zoom level 

[Actual result]:
- At 290% [step3] and at 280% [step4] zoom levels (displayed by location bar zoom controls and by Firefox menu zoom controls) no page zoom growth/shrinkage is reflected 

[Regression range]:
- This issue is not a regression, as it is reproducible all the way back to Firefox 4.0
Layout uses appunits to do its calculations, appunits are 1/60 of a CSS pixel, a CSS pixel being the pixels that pages specify sizes in. We zoom by changing the ratio of appunits to CSS pixels, but CSS pixels always remain a integer number of app units. So when going from 280% to 290% its quite likely that we round to the closest integer in both cases and it comes out as the same number. We can't implement every zoom ratio exactly, unless it can be represented as an integer in this way. This works well in most cases, but not in this edge case. This problem by itself isn't sufficiently bad that we'd change a fundamental way in which layout works.
Last Resolved: a year ago
Resolution: --- → WONTFIX
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