[okular] [Bug 474299] New: Typing long text in PDF forms extremely slow

[Louis Moureaux]

https://bugs.kde.org/show_bug.cgi?id=474299

            Bug ID: 474299
           Summary: Typing long text in PDF forms extremely slow
    Classification: Applications
           Product: okular
           Version: 23.04.3
          Platform: Neon
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: PDF backend
          Assignee: okular-devel at kde.org
          Reporter: m_louis30 at yahoo.com
  Target Milestone: ---

Created attachment 161510
  --> https://bugs.kde.org/attachment.cgi?id=161510&action=edit
Profile

SUMMARY
When editing a (specific) PDF form with a "free text" field, typing is
extremely slow with okular at 100% CPU usage (multiple seconds per character).
A quick profile shows Poppler::FormFieldText::setText(QString) as the main
culprit, apparently reloading fonts and querying FontConfig in the process.
Okular shouldn't call this function on every edit (and I suspect even for every
letter in the text, else it wouldn't be so slow).

STEPS TO REPRODUCE
1. Open a PDF form with a multiline text field (I can test patches/share mine
privately if needed)
2. Try to type some text

OBSERVED RESULT
Wait for 5 seconds per letter you type

EXPECTED RESULT
Editing test is instant

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.27
KDE Plasma Version: 5.27.7
KDE Frameworks Version: 5.108.0
Qt Version: 5.15.10
Kernel Version: 6.2.0-31-generic (64-bit)
Graphics Platform: Wayland

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