[Libreoffice-bugs] [Bug 140577] New: Writer in "Web" view mode becomes unresponsive and flickers when the window is resized or zoom is adjusted when a long document is displayed

[bugzilla-daemon at bugs.documentfoundation.org]

https://bugs.documentfoundation.org/show_bug.cgi?id=140577

            Bug ID: 140577
           Summary: Writer in "Web" view mode becomes unresponsive and
                    flickers when the window is resized or zoom is
                    adjusted when a long document is displayed
           Product: LibreOffice
           Version: 7.0.4.2 release
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: medium
         Component: Writer
          Assignee: libreoffice-bugs at lists.freedesktop.org
          Reporter: ckoresko at atlanticbb.net

Description:
When working with a long (>50000 words, say) document using Writer in Web view
mode, changing the zoom setting or resizing the window appears to trigger a
reflow operation of the whole document, which can render Writer unusable for
10s of seconds before the display stabilizes.  During this time it uses a full
CPU core.

Steps to Reproduce:
1. Load a long document into Writer.  The document should not have hard line
breaks.
2. Scroll to the end of the document.
3. Using the mouse, expand or shrink the width of the Writer window, or change
the Zoom setting.

Actual Results:
Document display flickers and CPU usage is high while Writer reflows the
document.    Eventually the display becomes stable and Writer can be used
normally until another change to the zoom setting or window geometry occurs.

Expected Results:
The display does not flicker.  Writer remains responsive.


Reproducible: Always


User Profile Reset: No


OpenGL enabled: Yes

Additional Info:
I suspect that this happens because the document is being redisplayed during
the calculated reflow operation.  I don't know the inner workings of Writer,
but perhaps it would be possible to first calculate the new locations of the
line-wraps in a background thread while the display continues to reflect their
positions with the previous zoom and window geometry.  I think the horizontal
scrollbar would have to be displayed if the window width is reduced or the zoom
is increased.  Then the display could be updated in one fast operation when the
reflow calculation thread exits.

Version: 7.0.4.2
Build ID: 00(Build:2)
CPU threads: 2; OS: Linux 5.10; UI render: default; VCL: gtk3
Locale: en-US (en_US.UTF-8); UI: en-US
Calc: threaded

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