v2.7-2.8 incorrectly render dashed line in FPDF/FPDI-created files.

[kanuay]

Attach (recommended) or Link to PDF file here:
https://kanuay.com/pdf/report/test.pdf

Configuration:

• Web browser and its version: Firefox Dev 88.0 & Safari iOS 14
• Operating system and its version: Windows 10 Latest & iOS 14
• PDF.js version: 2.7-2.8 (Both self-hosted and Firefox built-in)

Steps to reproduce the problem:
Lot of my file created by FPDF/FPDI
Open these files with PDF.js v2.7 or v2.8
All the fraction lines, radical signs, especially dashed lines are rendered incorrectly.

[timvandermeij]

If I recall correctly we did make some changes to the minimum line width code, although I'm not sure it's related to this.

[Snuffleupagus]

It seems that this regressed in two steps unfortunately, according to git bisect:

1. The rendering of the figure first broke in

   b3dccd66ab6f895aa4a721082fdac05277c6818b is the first bad commit
   commit b3dccd66ab6f895aa4a721082fdac05277c6818b
   Author: Calixte Denizet <[email protected]>
   Date:   Mon Jan 4 14:25:30 2021 +0100
   
       Enforce line width to be at least 1px after applied transform
        * add a comment to explain how minimal linewidth is computed.
        * when context.linewidth < 1 after transform, firefox and chrome
          don't render in the same way (issue #12810).
        * set lineWidth to 1 after transform and before stroking
          - aims fix issue #12295
          - a pixel can be transformed into a rectangle with both heights < 1.
            A single rescale leads to a rectangle with dim equals to 1 and
            the other to something greater than 1.
        * change the way to render rectangle with null dimensions:
          - right now we rely on the lineWidth set before "re" but
            it can be set after "re" and before "S" and in this case the rendering
            will be wrong.
          - render such rectangles as a single line.
   
   

2. The math symbols then became too bold in

   9754216c600a571f3cd10f819a88269b98ab2616 is the first bad commit
   commit 9754216c600a571f3cd10f819a88269b98ab2616
   Author: Calixte Denizet <[email protected]>
   Date:   Sat Jan 16 14:05:37 2021 +0100
   
       Fix zoom issue with too thin lines
        - aims to fix issue #12868: apply zoom factor to linewidth after setting it
          to 1.
        - only apply 1px-width when required
        - the sign of getSinglePixelWidth is used to know if 1px-width is required
   
   
   

/cc @calixteman

[calixteman]

I wonder if the correct approach would be to play with the transform to get a final correct rectangle (pixel after transform) instead of playing with the linewidth itself.

[calixteman]

I tried to fix the transformMatrix instead of fixing the line width and it seems to be the correct way to deal with this issue.
I'll submit a PR asap.