fix(gui): rebuild the download list in one pass when filtering or switching category - #670

Fixes #669.

Typing in the transfer-window text filter froze the GUI for seconds on a large queue — @ghysler reports ~10 s per keystroke with 10k downloads, with keystrokes stacking up. The shared-files filter is instant at the same scale.

Root cause

SetFilterText() re-evaluated every model item and called ShowFile(file, visible) for each. Every row that had to disappear went through RemoveItemData(), which does a full RebuildRowIndex() plus a RefreshFromRow() — O(n) per removal, so O(n²) for one keystroke. The shared-files filter is fast because it does the opposite: clears the model, re-appends the passing items, and finishes with a single FinishBulkLoad().

ChangeCategory() had the identical shape, so switching category on a large queue was equally slow. That wasn't reported, but it's the same bug and is fixed here.

Fix

RebuildVisibleList() clears, appends every item passing the current category + text filter, and finishes with one FinishBulkLoad() (one SetItemCount, one index rebuild, one sort) — O(n log n) and a single repaint. ShowFileList(), the filter and ChangeCategory() all route through it, so the three paths no longer duplicate the visibility loop. ShowFilesCount(diff) delegates to a new SetFilesCount(count) so the rebuild can set the count absolutely without duplicating the label update.

De-duplicating the filter

The two lists had grown a filter each: identical m_filterText members, identical SetFilterText() bodies, and byte-identical PassesTextFilter() implementations differing only in the parameter type (const CPartFile* vs const CKnownFile*).

That now lives once in CMuleVirtualListCtrl — m_filterText, SetFilterText() and a string-based MatchesFilter(), plus a RebuildFilteredView() hook each list overrides with its own rebuild. MatchesFilter() takes a name rather than a file so the generic control keeps knowing nothing about what its rows represent, and each subclass is left owning only what is genuinely specific: which model it walks and what it matches on. (CSearchListCtrl's filter is a regex on the non-virtual base and is untouched.)

Selection preservation

A clear-and-re-append rebuild renumbers every row, and the virtual control tracks selection and focus by row index — so both would be dropped, where the incremental path kept them for free by leaving surviving rows untouched.

CMuleVirtualListCtrl grows SaveSelection() / RestoreSelection() for this. SortList() already did the same save-and-reapply inline and now shares them, and so does CSharedFilesCtrl::ShowFileList() — which fixes a pre-existing bug of its own: editing the shared-files filter dropped the selection.

Testing

Built amule and amulegui clean. The algorithmic change is the substance here; a 10k-download reproduction wasn't available locally, so a real-world check on the reporter's setup is welcome before this is relied on.