[kmail2] [Bug 361834] New: Incorrect calculation of repaint region when message display pane is close to requiring scroll bars?

[Kjetil Kilhavn via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=361834

            Bug ID: 361834
           Summary: Incorrect calculation of repaint region when message
                    display pane is close to requiring scroll bars?
           Product: kmail2
           Version: 5.1.3
          Platform: openSUSE RPMs
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: UI
          Assignee: kdepim-bugs at kde.org
          Reporter: kjetil at kilhavn.no

An email message that in its HTML form contains a background image which is an
external resource leads to a display anomaly when loading external resources.
The problem occurs only for specific window sizes, so I suspect it is related
to calculation of the region that must be repainted.

I accidentally discovered that if I turn on display of message structure
(displayed below message pane) the problem does not occur! So I experimented a
little more with the message opened in a separate window and discovered that
the problem did not occur then either. However; it turns out that it depends on
the window size - so when changed the size of the separate message window to
match the message area size in KMail the problem occurred again. It did not
occur for a smaller window, nor for a larger window. 
I suspect that with a larger screen I would be able to reproduce the problem
also with the message structure displayed.

Reproducible: Always

Steps to Reproduce:
If required, change KMail settings to display plain text email by default and
display HTML status bar in message window before attempting to reproduce the
problem.

1. Load attached message into KMail
2. Switch to HTML version
3. Load external resources

Actual Results:  
The right edge of the window (where the scroll bar was before loading external
resources) is not repainted correctly. It shows the mail header and parts of
the background - from the size distortion it seems there is an initial
rendering run, and the scroll bar area shows the result of this rendering.

Expected Results:  
The message should have displayed without a strange artefact in the scrollbar
area.

I am setting the severity to Normal instead of Minor because I hope there can
also be some performance gained from fixing this bug - assuming a fix leads to
a 50% reduction in rendering runs (from 2 to 1).

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