improved perfs and event management

src/ui_app.hpp

@@ -743,6 +743,12 @@ struct App {
   /// it keeps for a second latch (it is counting retries, not time).
   int dbg_events_step = 0;
   int dbg_events_wait = 0;
+  /// The event row the probe selected before it activated one, and how many rows were selected.
+  /// Both are read back *after* the activation: a row pointer that is no longer the list's row 0,
+  /// or a smaller count, says the activation rebuilt the list and so threw away the user's scroll
+  /// position and their comparison selection.
+  GtkListBoxRow* dbg_events_row0 = nullptr;
+  int dbg_events_selected = 0;
   /// Which step of the plot-context-menu probe runs next (`UDPSCOPE_PROBE_ROWMENU_MS`), and the
   /// popover it is measuring.
   int dbg_rowmenu_step = 0;
@@ -1097,7 +1103,13 @@ constexpr double kWirePick = 6.0;
 constexpr int kZoomGestureSteps = 8;
 constexpr int kZoomWindowSteps = 8;
 constexpr int kEventThumbW = 138;
+/// The row thumbnail's height floor and ceiling. A floor and not a fixed size: the stored preview is
+/// `44 px` per signal, so its proportions depend on how many signals the event has, and a row shows
+/// it at *its* aspect -- a one-signal event is a short row and a six-signal one a tall one. The
+/// ceiling is what keeps a wide event from making a row taller than the pane; the full picture is a
+/// hover away.
 constexpr int kEventThumbH = 46;
+constexpr int kEventThumbMaxH = 104;
 inline int g_stats_phase = 0;
 inline std::vector<std::uint64_t> g_stats_n_before;
 inline double g_trigrenum_edge = 0.0;
@@ -1333,6 +1345,15 @@ GdkTexture* scale_texture(GdkTexture* source, int width, int height);
 EventThumbCache* event_thumb_cache(App& app);
 GdkTexture* event_preview_texture(App& app, const std::string& path);
 void refresh_events_pane(App& app);
+/// Re-marks which row is the event currently on screen, **without rebuilding the list**.
+///
+/// The mark is the only thing about a row that a replay changes -- the picture and the numbers
+/// are the file's -- and rebuilding the list to restate it threw away the scroll position and
+/// the selection with it, so reaching the thirtieth event meant scrolling to it again after
+/// every click, and a selection made for a comparison was lost by the act of looking at one of
+/// its members. Call this for a change of what is on screen, and `refresh_events_pane()` only
+/// for a change of what is *cached*.
+void mark_events_on_screen(App& app);
 void on_event_row_activated(GtkListBox*, GtkListBoxRow* row, gpointer user);
 GtkWidget* build_events_pane(App& app);
 gboolean tick(gpointer user);