Replace `QPlainTextEdit` log viewer with virtualized `QListView`-based architecture

[buhtz]

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Problem description

The current log viewer is implemented using QPlainTextEdit to display rsync log files. This approach does not scale to large log files (observed up to ~700 MB). Users reporting UI freezes and crashes (#456, #867, #890, #2530). The root cause is likely that the entire log content is loaded and rendered as a single text document.

Goals

• Support very large log files (up to 700 MB)
• Keep UI responsive during load, scroll, and filtering
• Reduce RAM use
• Support enriched log visualization (icons, colors, severity indicators) (Snapshot log view: Add help for rsync itemize-changes 11-char output #1579, please explain all the abbreviations in the log #1218)

Proposed solution

This is a quick n dirty idea, not a fixed requirement. It is open for discussion.

Replace the current text-based log viewer with a Qt Model/View architecture using QListView.

Core design

• QListView as display widget
• custom QAbstractListModel to manage log entries
• Each log line is represented as a lightweight item (no full text buffer in memory)
• Qt handles virtualized rendering (only visible rows)

Data handling

• Log file is not fully loaded into memory
• Background worker builds index of file: byte offsets per line
• Optional parsing of metadata (timestamp, severity level, rsync status)
• Lazy loading of lines via file seek operations

Filtering

• Filtering is applied at model level, not UI level
• QSortFilterProxyModel or precomputed filtered index lists
• No full file re-parsing on filter changes

Rendering in UI

• QStyledItemDelegate for custom rendering:
  • severity-based colors (ERROR / WARN / INFO)
  • icons per log level
  • structured layout (timestamp + message)
• Maybe highlighting of filter matches

Architecture overview

• Worker thread:
  • parses file
  • builds line index (offset table)
  • extracts metadata
• QAbstractListModel:
  • exposes indexed log lines
  • performs lazy file reads via seek()
• QListView:
  • virtualized rendering of visible items only
• Delegate:
  • handles visual representation and styling