441634 - multicolumn container width and gap miscalculated in pt (only?)

Status: NEW

(Core :: Layout: Block and Inline, defect)

Description

[Zack Weinberg (:zwol)]

Attachment: wrong container width testcase

In the attached testcase, the two rows should appear identical, modulo rounding errors.  Instead, in the second row, the bottom segment of the blue box is drawn outside the gray box, and there are large gaps between segments of the blue box.

This rendering error appears to happen only for certain combinations of value and units in the width specifications.  I can get it to go away by using in instead of pt, or a value in pt that corresponds to a round number of in.  It doesn't seem ever to happen for units in px, though. 

This is a distillation of a reftest that fails for me but not on the tinderboxes, so I will attach a screenshot in case nobody else can reproduce it.

Comment 1

[Zack Weinberg (:zwol)]

Attachment: screen shot of bad rendering

Comment 2

[Zack Weinberg (:zwol)]

for the record, the reftests that are failing for me were introduced with the fix for bug 379349.

Comment 3

[Zack Weinberg (:zwol)]

See also bug 441644, which may be another symptom of the same problem.

Comment 4

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

Does this still happen if you set (in about:config) the layout.css.dpi pref to 96 or 120 or 144?  If not, what value matches your normal running conditions under which it fails?

Comment 5

[Zack Weinberg (:zwol)]

I tried a bunch of values of layout.css.dpi; most values give me a correct rendering, but setting it to 98, 116, or 143 reproduces the bug.  (98 is what my X server says, and therefore, I assume, what it's using when the pref is set to 0.) Changing the pref by 1 either way makes the problem vanish.  Rounding error under very specific conditions?

Comment 6

[Zack Weinberg (:zwol)]

To be clear, all the values you suggested (96, 120, 144) gave a correct rendering.