1964005 - Text rendering garbled near the end with long text files

Status: NEW

(Core :: Layout: Text and Fonts, defect)

Description

[Bojan Smojver]

Attachment: Screenshot From 2025-04-25 07-52-28.png

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:138.0) Gecko/20100101 Firefox/138.0

Steps to reproduce:

Open a very large file (e.g. https://kojipkgs.fedoraproject.org//packages/firefox/138.0.1/1.fc42/data/logs/x86_64/build.log), then scroll all the way down to the end of it. The text near the end will appear garbled (see screenshot).

This affect pretty much any current version of Firefox in F42 (and F41, F40) on the machine I use (ThinkPad T450s). Not quite sure whether this is a problem in the Firefox itself or some underlying component, so opening here first. Desktop environment is gnome. Fonts are default.

Reproducible: Always

Steps to Reproduce:

1. Open a very large text file (e.g. a build log in koji/copr).
2. Scroll to the very end.
3. Observed incorrectly rendered text.
   Actual Results:
   Text rendered incorrectly.

Expected Results:
This works fine on MacOS with the same versions of Firefox.

Additional Information:
Please see attached screenshot.

Actual results:

Garbled text.

Expected results:

Works on other platforms.

Comment 1

[Daniel Holbert [:dholbert]]

Thanks for the bug report. For what it's worth, I can't reproduce on Ubuntu 24.04 (on a display with 100% pixel scaling, i.e. no HiDPI). I can trigger a version of this bug if I zoom in all the way though with full-page-zoom (500% scale factor).

The last few lines of the log are supposed to look like this:

Wrote: /builddir/build/RPMS/firefox-debuginfo-138.0.1-1.fc42.x86_64.rpm
RPM build warnings:
    Duplicate build-ids /builddir/build/BUILD/firefox-138.0.1-build/BUILDROOT/usr/lib64/firefox/firefox and /builddir/build/BUILD/firefox-138.0.1-build/BUILDROOT/usr/lib64/firefox/firefox-bin
    absolute symlink: /usr/lib64/firefox/dictionaries -> /usr/share/hunspell
    Duplicate build-ids /builddir/build/BUILD/firefox-138.0.1-build/BUILDROOT/usr/lib/debug/usr/lib64/firefox/firefox-138.0.1-1.fc42.x86_64.debug and /builddir/build/BUILD/firefox-138.0.1-build/BUILDROOT/usr/lib/debug/usr/lib64/firefox/firefox-bin-138.0.1-1.fc42.x86_64.debug
Child return code was: 0

But in the screenshot and in my local testing with 500% scale factor, there's other text at the end of the log instead (and various other character-positioning glitches).

A few observations:

• The text file here (linked in comment 0) is 145MB in size.
• When I view the file at the default 100% zoom level, window.document.documentElement.scrollHeight is 12749200 (which nearly reaches nscoord_MAX in CSS pixels, which is the upper limit of lengths that we reliably track for layout). (That's 2^30-1 nscoord units, which are 1/60th of a pixel, at 100% zoom/pixel-scaling level -- so that threshold is 17895697 css pixels).
• That means full-page-zoom and/or HiDPI could easily push us over that nscoord_MAX limit at which point stuff is liable to get garbled.

So this is sort of a version of bug 265084. Gecko's layout representation isn't robust for documents of this size.

Comment 2

[Jonathan Kew [:jfkthame]]

See also bug 1726431, where we observed that rendering became erratic when the height of the content exceeds 16M device pixels.

Comment 3

[Bojan Smojver]

I was testing on a 4K external monitor and I think the scaling I set there was not 100%, but something like 125% (from memory - not on that machine now).

Comment 4

[Bojan Smojver]

It is indeed 125% on that monitor.

Comment 5

[Monica Chiorean]

I was able to reproduce the issue on Ubuntu24.04 using Firefox build 140.0a1 with 100%scale.
Marking issue as new.