172683 - Problem with layout of Arabic lam-alef ligatures and nsRenderingContext{GTK|Xlib}::GetTextDimensions()

Status: RESOLVED WORKSFORME

(Core :: Layout, defect)

Description

[Simon Montagu :smontagu]

I came across this problem working on the decoder for ISO-8859-6.8x fonts (bug
78201). Arabic lam-alef ligatures are split into two glyphs in this encoding,
and this caused layout problems.

setting MOZILLA_GFX_DISABLE_FAST_MEASURE=1 in my environment removed the problems.

Steps to reproduce:

Apply http://bugzilla.mozilla.org/attachment.cgi?id=101636&action=view
Install an ISO-8859-6.8x font or fonts (e.g.
http://www.langbox.com/AraZilla/linux/arafontfull-1.2-4.i386.rpm)
Load the testcase which I will attach shortly.

Expected/actual results: described in the testcase.

Comment 1

[Simon Montagu :smontagu]

Attachment: The test case

Comment 2

[Roland Mainz]

I need a little bit help here - the fast text measuring code is disabled on
non-Linux/x86 platforms (see
http://lxr.mozilla.org/seamonkey/source/gfx/src/gtk/nsRenderingContextGTK.cpp#213)
and I am mainly doing development on Solaris/SPARC where the problem won't
occur...

Comment 3

[Roland Mainz]

rbs:
Is there any known interaction problem with the |GetTextDimensions()|-code and
BiDi text layout ?

Comment 4

[Roland Mainz]

smontagu:
Can you try to turn-off |NS_RENDERING_HINT_FAST_MEASURE| selectively for the
following two codepoints and check if the problem goes away:
a)
http://lxr.mozilla.org/seamonkey/source/layout/html/base/src/nsTextFrame.cpp#4651
b)
http://lxr.mozilla.org/seamonkey/source/layout/html/base/src/nsTextFrame.cpp#4958

Comment 5

[Simon Montagu :smontagu]

>Can you try to turn-off |NS_RENDERING_HINT_FAST_MEASURE| selectively for the
>following two codepoints and check if the problem goes away:
>a)
>http://lxr.mozilla.org/seamonkey/source/layout/html/base/src/nsTextFrame.cpp#4651

Turning off NS_RENDERING_HINT_FAST_MEASURE here makes the problem go away

>b)
>http://lxr.mozilla.org/seamonkey/source/layout/html/base/src/nsTextFrame.cpp#4958

Turning it off here causes a hang :-(

Comment 6

[Roland Mainz]

Filed bug 173013 ("Need workaround for problem with layout of Arabic lam-alef
ligatures and nsRenderingContext{GTK|Xlib}::GetTextDimensions()") to get a
workaround per feedback in comment #5 ...

Comment 7

[Simon Montagu :smontagu]

Reassigning to myself for now. rbs, do you have any thoughts on this?

Comment 8

[rbs]

I am not sure what is happening. You might want to debug to see the new
interaction that your patch in comment 0 is producing. From there on, there will
hopefully be more data as to what is causing the problem.

Comment 9

[Simon Montagu :smontagu]

This was fixed by bug 188759.