Do we need Qt4 for LyX 2.4.0?

[Jean-Marc Lasgouttes]

[this is a reboot of a discussion oat developpers at lyx.org. Sorry for the 
repetition]

Hi there,

I am currently fighting with Qt4 for getting correct line-breaking [*].

I had to create a Ubuntu 18.04 virtual machine to have a working Qt4, 
and I do not know how this will work in 2026 when we will still be 
waiting for this LyX 2.5.0 release that works with Qt6 (or 7?) only.

So I am wondering what is the use case for Qt4 in LyX 2.4. Ubuntu 18.04 
has a perfectly working Qt 5.9.5. Who are the people who rely on Qt4 now 
(considering the big delay that we have had with LyX 2.4.0) ? I was one 
of them, I am not anymore.

One possible plan could be to say that LyX requires Qt 5.6 (which was 
considered at some time as the first working Qt5 version if I remember 
well), which was released in 2016.

The other plan is to continue to pretend that LyX compiles with gcc 4.9 
and and Qt4 for the 2 to 4 years to come. I do not know who is going to 
do the minimal work to ensure that this is somewhat true, but it has a 
cost. Moreover the work towards Qt6 (which will undoubtlessly happen) 
will make us stretch a bit more.

So is there one person on this list who needs Qt4 support? I can also 
ask on lyx-users if needed.

Thoughts?

JMarc

[*] basically Qt4 thinks that zero-width characters do not have 
zero-width in monospace font. This creates bugs with the line breaking 
code, which relies on WORD JOINER and LRO/RLO overrides, which will be 
difficult/impossible to work around.