Hover circle misplaced on a radio button when zooming to 1000% level

[bogdanmaris]

Affected versions

• 81.0a1 (2020-08-18)

Affected platforms

• Windows 10 64bit
• Ubuntu 18.04 64bit
• macOS 10.15

Steps to reproduce
pdfjs.renderInteractiveForms - true

1. Launch Firefox
2. Access a PDF form with fillable content (eg: https://pdf.ninja/interactiveform_enabled.pdf)
3. Scroll down to the radio buttons
4. Zoom to level 1000%
5. Hover over the radio button

Expected result

• Hover circle is within the borders of the radio button

Actual result

• Hover circle is misplaced a bit when zoomed in

Additional notes

• S4 as suggested severity, only noticeable when zooming to 1000%.
• Here is a .gif showing the issue:

[timvandermeij]

Note that this also applies to checkboxes as the PR above suggests.

[serdnab]

I am interested in fixing this.
The problem is the rounding here.
When the canvas has more rounded pixels than the canvas style or vice versa, misalignment occurs.
Why is it necessary for the canvas and canvas style to be multiples of the approximation of
fractions and not just an integer?
Rounding to just an integer fixes the problem.

[Snuffleupagus]

The problem is the rounding here.

Unfortunately I believe that that's most likely a red herring, since the thing that becomes slightly wrong when CSS-only zooming is being used is probably the annotationLayer-positioning and not necessarily the canvas itself (since it works just fine if "regular" zooming is force-enabled, even at very large values).

Why is it necessary for the canvas and canvas style to be multiples of the approximation of fractions and not just an integer?

Please refer to PR #6552; basically it's necessary to avoid issues/glitches on HiDPI screens. Hence removing that would very likely re-introduce other issues, and again it's probably not the actual underlying issue here unfortunately.

[serdnab]

How do you force-enable regular zooming?

[Snuffleupagus]

How do you force-enable regular zooming?

You'd have to increase the maxCanvasPixels option, however that's probably not entirely relevant here (since this issue just concerns the CSS-only zooming mode).
Thinking about this some more, I suppose that another possible cause of the bug could be that the PDFPageView.cssTransform-method in particular isn't placing the canvas entirely correct (e.g. not handling fractional widths/heights correctly). However, in order to fix this issue I think that the very first step is to figure out exactly why the canvas/annotationLayer positions differ specifically when CSS-only zooming is being used.

[serdnab]

When you open the file at 600% zoom, before applying any zoom, don't you see a misalignment?