A problem I'm having with my HiDPI display, remote X, and (X) cursors
When I set up my HiDPI display on my home Linux machine, I had to do some wrestling with general DPI and scaling settings but after that most everything just worked and I didn't think about it. Due to world and local events, I spent a chunk of today getting set up for an extended period of working from home, including getting my work exmh configured to display properly over remote X on my HiDPI home display.
(Exmh is one of the things that I really miss when I don't have X across the network, and my current DSL link is actually fast enough to make it useful for reading email. If I'm going to be working from home for an extended period of time, I need a good email environment so it was worth the effort to see if exmh could run decently over my DSL link.)
This worked in general (with a few mistakes along the way), but
after using exmh for a while I realized that the (X) mouse cursors
that I was seeing when my mouse was over the exmh windows were
unusually and suspiciously small, as if they hadn't been scaled up
to HiDPI levels. At first I thought that this was a TCL/TK issue, but then I looked
at the mouse cursors I was seeing in other programs run over the
remote X connection (such as
xterm), I saw the same issue. My local
xterm windows have
a mouse cursor that's the right size (roughly the size of a capital
letter in the
xterm), but an
xterm on our Ubuntu machines run
over remote X has one that's half the correct size. The same is
true of the cursors in exmh, GNU Emacs, and sam.
(In the process of writing this entry, I checked my office Fedora machine and to my surprise, these programs all work correctly there over a remote X connection.)
X mouse cursors are a very old thing and in the way of X they've gone through a number of evolutions over the years (and then things like GUI toolkits and theming added extra layers of fun). The result is relatively opaque and underdocumented, especially if what you care about is basic X stuff like xterm and TCL/TK (for natural reasons, most people focus on writing about full scale desktops like GNOME and KDE). I found a variety of things on the Internet, some of which didn't work for me and some of which aren't feasible because the remote machines are multi-user ones and not everyone doing remote X to it has a HiDPI display (I won't when we go back to work, for example).
These days, there are apparently cursor themes, as discussed a bit
in the Gentoo wiki
and this article
(and see also).
Some basic X programs in some environments pay attention to this,
through both X resources settings and environment variables (per
the Arch wiki), but
on our Ubuntu machines the various X programs seem to ignore the
environment variables (although this stackoverflow answer talks
On Fedora the
$XCURSOR_SIZE environment variable and so on does
Our Ubuntu machines have the libxcursor shared library installed
(as 'libXcursor.so') and a running
xterm uses it, but they don't
seem to have any X cursor files installed (we don't have the
xcursor-themes package present, for example). This may mean that
our Ubuntu machines are forced to fall back to some very old X
protocol thing and that X protocol thing only has one size, that
being the tiny non-HiDPI cursors. My Fedora machines do appear to
have cursor themes installed in stuff under /usr/share/icons, and
it looks like if I copy the right one ('Adwaita') to our Ubuntu
machine and set
$XCURSOR_THEME, my exmh,
xterm, and so on work right.
(I think that setting these environment variables in general is harmless for non-HiDPI sessions, because I believe that the X cursor library magically picks the right size based on your display DPI.)
I suspect that this is a sign that our Ubuntu machines don't really have all of the X related packages that they should have in order to make modern X programs happy in modern X environments (which definitely include HiDPI screens). I'm not sure what additional packages we need, though, which means that I have a new project. In the mean time, writing this entry has gotten me to do enough research to find a workaround for now.