Regardless, this is a textbook example of a browser + platform bug 
since FF both:

1) does NOT display this issue on other platforms
2) does NOT display this issue with other versions on the same platform

Nothing we can do here.  A broken browser is a broken browser.

I cannot see any segfaults on my PC with Firefox or Chrome.

FWIW, I saw this issue on the server immediately after I tried to 
right-click. No idea whether it is related or not.

[ 9066.833088] chrome[3091]: segfault at 5eccc0b0 ip 5eccc0b0 sp 
bfb3531c error 4 in libGL.so.1.2[62982000+52000]

OK - I can reproduce but ONLY on Linux (Ubuntu) and ONLY with specific 
browsers (FF 18.0.2 shows this issue, but FF 16 doesn't).  So this is 
a browser and/or linked library issue specific to Linux and is not a 
Horde/IMP issue.

Short of you tracing the code and telling us exactly *why* the click 
event is not being handled correctly by the ContextSensitive 
javascript code, there is nothing more we can do.

I can point you to horde/js/contextsensitive.js - the 
_rightClickHandler method.  That method is triggered on a right click 
to the message row.  The e parameter contains the Event and 
'e.element()' contains the DOM element that was clicked on (which will 
be the child of the message row).

Well, i made some tests before and created really simple HTML e-mail 
(it must be text/html) with numbered lines. When i changed size of the 
preview so only one message from message list was displayed, problem 
appeard right after i scrolled after the first line of e-mail. When 
preview was small and/or opened message was high in the list, i had to 
scroll several lines - at least exactly the size of the length between 
opened message and top of the preview. Firefox 18.0.2 on linux. It's 
always reproducable, i can give you any other details, just ask what 
you need.

Yes

And you are right-clicking on the *selected/opened* message, right? 
Cos right click anywhere else is working fine.

Except I can minimize the message list to, say, 2 entries and display 
an HTML message that is 2000 lines long, and scroll all the way to the 
bottom, and right-clicking still works fine on the message list.

In other words: The DOM ID previewPane DIV does extend to the top of 
the browser (and above) when scrolling to the bottom of a longer HTML 
message.  But this is irrelevant because the message rows are 
contained in the 'msglist' DIV, which is a floating element so even if 
the previewPane DIV occupies the same space, it will be displayed 
UNDER the float - not over.

You are going to need to get deeper into the browser fundamentals to 
determine why the right-click DOM event is being triggered/handled by 
this DIV instead of the msglist row.

Ok, i found out where exactly the problem is. Problem appears when i 
scroll down the message in preview long enough so the hidden part 
above is as long (or longer) as the size between the preview and 
selected message. Opened message probably overlaps the message list so 
i'm not right clicking into the message list but into the opened 
message. Do you know what i mean?

I don't doubt that it's happening, but I can't reproduce so I am 
unable to debug/fix any further.  Unless you can provide a solution, 
or another dev can reproduce, this ticket will be stalled.

Attaching two images of the same message with different level of 
scroll as a proof that this is really happening to me in all tested 
browsers :) Looks like only HTML messages are affected. Sometimes i 
have to scroll almost down, sometimes only few lines.

I can't reproduce with that sample message.

Michael, i sent you an example by email.

You will need to as I can't reproduce.  Does it only happen with HTML 
messages?  Probably an issue with the z-index "cap" we need to put 
over the IFRAME to allow things like click/drag to work properly.

I just noticed that this does not affect all messages. I can privately 
send an example.

I'm experiencing a strange right-click bug in dynamic mode (tried 
Firefox 18.0.2 and Chrome 25). When preview is enabled and you open a 
long message, right-click on (selected/opened) message in message list 
is not working properly:
  - when you are at the top of the opened message (using scrollbar at 
the right side), right-click on selected message in message list works 
ok

  - when you scroll the message down sufficiently (at the bottom, for 
example but message must be long enough), right-click stops showing 
IMP menu and shows standard browser right-click menu instead

Always reproducable.