fix: row titles mean their mnemonics instead of drawing the underscore

Fifteen display strings in the two preference forms were written as GTK mnemonics -- `_Directory`,
`_Server`, `Multicast _group`, `_Theme` and the rest -- but the property that makes GTK read them was
never set, so GTK drew the underscore literally: `_Directory` in the row, not a D with a line under it.

Two switches are involved, FALSE by default in each case. `GtkLabel`/`GtkButton` take `use-underline`;
the rows take `AdwPreferencesRow:use-underline`, "whether an embedded underline in the title indicates a
mnemonic". The two buttons in the source form already set it (so their mnemonics worked and showed no
underscore); the thirteen rows did not. All thirteen now do.

The row classes that carry the property are not all related -- AdwEntryRow has its own, while AdwSpinRow,
AdwComboRow and AdwSwitchRow get it through AdwActionRow -- so the probe reads it by name on a GObject
(g_object_class_find_property, then g_object_get) rather than casting to one row type; a cast to a
`GtkWidget*` where the type is only forward-declared does not compile at all.

`_Cancel` and `_Connect` both claimed C in the same form, so Alt+C could only reach one of them: that
marker was unreachable, which is worse than absent because it is in the source to be trusted. `_Connect`
becomes `Conn_ect`, freeing C for Cancel.

[prefs-probe] reads the count (4/4) on the real widgets; check_ui validates the property against the GIR
for every object in every .ui file, inherited properties included.

docs/claude/invariants-gtk-and-chrome.md

@@ -280,3 +280,21 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   re-enter a dead window. GTK does not promise to take a transient down with its parent, and a window
   whose handlers point into a returned stack frame is how this project already earned one
   use-after-free.
+
+- **A `_` in a label is a mnemonic only on a widget that was asked for one.** GTK reads the underscore as
+  "the next letter is this widget's accelerator, and draw it underlined", so wherever the widget has not
+  been asked, GTK draws the underscore *literally* -- which is what `_Directory`, `_Server`, `Keep _events`
+  and eleven more titles did in the two preference forms, for as long as they existed. Two different
+  switches, FALSE by default in both cases: `GtkLabel`/`GtkButton` take `use-underline`, and the rows take
+  **`AdwPreferencesRow:use-underline`** ("whether an embedded underline in the title indicates a mnemonic").
+  The row classes that carry it are not all related -- `AdwEntryRow` has its own, while
+  `AdwSpinRow`/`AdwComboRow`/`AdwSwitchRow` get it through `AdwActionRow` -- so `row_underlines()` asks by
+  name on a `GObject` (`g_object_class_find_property`, then `g_object_get`) instead of casting to one row
+  type: casting a row whose type is only forward-declared in that translation unit does not even compile.
+  `test-ui` is what catches the property being set on a class that does not have it -- it validates every
+  property against the GIR, inherited ones included.
+- **A mnemonic is unique per window, and the first match wins.** `_Cancel` and `_Connect` both claimed C in
+  one form, so `Alt+C` could only ever reach one of them and the other's marker was unreachable -- worse
+  than no mnemonic, because the underscore is there in the source for a reader to trust. `Conn_ect` freed
+  it. Rows on different pages of a preferences dialog may share a letter: an unshown page cannot be
+  activated.

docs/claude/probe-readings.md

@@ -1143,3 +1143,16 @@ copy and never writes is exactly the failure this has to catch.
   default path back into the field and presses *Apply*, and the file must end up with **no**
   `event_cache_dir` at all. A path in the file is a user's choice, and typing the default is not one --
   pinning it there would stop the setting following the cache directory it is named after.
+
+## `mnemonics=` in the two preference probes
+
+```
+[prefs-probe]  mnemonics=4/4
+[sources] form mnemonics=8/8
+```
+
+How many of the dialog's rows have `use-underline` set -- the property that makes a `_` in a row's title a
+mnemonic. It is FALSE on `AdwPreferencesRow` by default, which is why these titles were drawn with a
+literal underscore; anything below the full count is that bug back. Read by property name rather than
+through one row type, for the reason in `invariants-gtk-and-chrome.md`. The form's line appears only when
+`UDPSCOPE_PROBE_NEWHOST` drives the form.

src/source_dialog.cpp

@@ -642,6 +642,13 @@ gboolean pick_probe(gpointer user) {
     gtk_editable_set_text(GTK_EDITABLE(c.form_note), "made by the new-host probe");
     adw_spin_row_set_value(c.form_buffer, 32);
     adw_switch_row_set_active(c.form_relative, TRUE);
+    // 8/8: the form's row titles are mnemonics (`_Server`, `_Port`, `Multicast _group`, ...). The two
+    // buttons already asked for it; the rows did not, and drew their underscores.
+    std::fprintf(stderr, "[sources] form mnemonics=%d/8\n",
+                 (row_underlines(G_OBJECT(c.form_server)) + row_underlines(G_OBJECT(c.form_port)) +
+                  row_underlines(G_OBJECT(c.form_group)) + row_underlines(G_OBJECT(c.form_iface)) +
+                  row_underlines(G_OBJECT(c.form_name)) + row_underlines(G_OBJECT(c.form_note)) +
+                  row_underlines(G_OBJECT(c.form_buffer)) + row_underlines(G_OBJECT(c.form_relative))));
     std::fprintf(stderr,
                  "[sources] form: name='%s' note='%s' buffer=%.6g relative=%d "
                  "connect_sensitive=%d\n",

src/ui/preferences.ui

@@ -32,12 +32,14 @@
             <child>
               <object class="AdwEntryRow" id="dir_row">
                 <property name="title" translatable="yes">_Directory</property>
+                <property name="use-underline">True</property>
                 <property name="tooltip-text" translatable="yes">One directory in which every source gets a folder of its own, never one directory between two sources</property>
               </object>
             </child>
             <child>
               <object class="AdwSpinRow" id="keep_row">
                 <property name="title" translatable="yes">Keep _events</property>
+                <property name="use-underline">True</property>
                 <property name="subtitle" translatable="yes">Per source; 0 keeps every event, which is a decision about the disk</property>
                 <property name="adjustment">
                   <object class="GtkAdjustment">
@@ -62,12 +64,14 @@
             <child>
               <object class="AdwSwitchRow" id="relative_row">
                 <property name="title" translatable="yes">_Relative time</property>
+                <property name="use-underline">True</property>
                 <property name="subtitle" translatable="yes">Take the plot's time from the sample rate and the packet counter rather than from the stream's own clock, so arrival and clock jitter cannot move a sample; absolute time is still kept for export. Read at start-up, because the buffers are planned before any source is known.</property>
               </object>
             </child>
             <child>
               <object class="AdwComboRow" id="theme_row">
                 <property name="title" translatable="yes">_Theme</property>
+                <property name="use-underline">True</property>
                 <property name="subtitle" translatable="yes">What the interface looks like; auto follows the system</property>
               </object>
             </child>
@@ -84,6 +88,7 @@
             <child>
               <object class="AdwButtonRow" id="reset_row">
                 <property name="title" translatable="yes">_Reset to defaults</property>
+                <property name="use-underline">True</property>
                 <property name="start-icon-name">view-refresh-symbolic</property>
               </object>
             </child>

src/ui/source-form.ui

@@ -38,7 +38,7 @@
             </child>
             <child type="end">
               <object class="GtkButton" id="form_apply">
-                <property name="label" translatable="yes">_Connect</property>
+                <property name="label" translatable="yes">Conn_ect</property>
                 <property name="use-underline">1</property>
                 <property name="receives-default">1</property>
                 <property name="css-classes">suggested-action</property>
@@ -59,11 +59,13 @@
                     <child>
                       <object class="AdwEntryRow" id="server_row">
                         <property name="title" translatable="yes">_Server</property>
+                        <property name="use-underline">True</property>
                       </object>
                     </child>
                     <child>
                       <object class="AdwSpinRow" id="port_row">
                         <property name="title" translatable="yes">_Port</property>
+                        <property name="use-underline">True</property>
                         <property name="adjustment">
                           <object class="GtkAdjustment">
                             <property name="lower">1</property>
@@ -77,11 +79,13 @@
                     <child>
                       <object class="AdwEntryRow" id="group_row">
                         <property name="title" translatable="yes">Multicast _group</property>
+                        <property name="use-underline">True</property>
                       </object>
                     </child>
                     <child>
                       <object class="AdwEntryRow" id="iface_row">
                         <property name="title" translatable="yes">_Interface</property>
+                        <property name="use-underline">True</property>
                       </object>
                     </child>
                   </object>
@@ -93,16 +97,19 @@
                     <child>
                       <object class="AdwEntryRow" id="name_row">
                         <property name="title" translatable="yes">_Name</property>
+                        <property name="use-underline">True</property>
                       </object>
                     </child>
                     <child>
                       <object class="AdwEntryRow" id="note_row">
                         <property name="title" translatable="yes">N_ote</property>
+                        <property name="use-underline">True</property>
                       </object>
                     </child>
                     <child>
                       <object class="AdwSpinRow" id="buffer_row">
                         <property name="title" translatable="yes">_Buffer MB</property>
+                        <property name="use-underline">True</property>
                         <property name="subtitle" translatable="yes">sample buffer per source, applied when this source is opened</property>
                         <property name="adjustment">
                           <object class="GtkAdjustment">
@@ -117,6 +124,7 @@
                     <child>
                       <object class="AdwSwitchRow" id="form_relative_row">
                         <property name="title" translatable="yes">_Relative time</property>
+                        <property name="use-underline">True</property>
                         <property name="subtitle" translatable="yes">plot time from the packet counter and the declared rate, instead of the stream's own time signal</property>
                       </object>
                     </child>

src/ui_app.hpp

@@ -13,6 +13,7 @@
 
 #pragma once
 
+
 #include <gtk4/gtk_plot_area.hpp>
 
 #include "acquisition.hpp"
@@ -33,6 +34,8 @@
 #include "status_text.hpp"
 #include "waveform.hpp"
 
+
+
 #include <adwaita.h>
 #include <gtk/gtk.h>
 

src/ui_prefs.hpp

@@ -29,6 +29,23 @@
 
 #include "settings.hpp"
 
+/// Whether a row asks for `use-underline` -- the property (default FALSE) that makes a `_` in a row's title
+/// a mnemonic. Without it GTK draws the underscore as a literal character, which is what `_Directory` and the
+/// other row titles did.
+///
+/// Asked for by *name* on a plain `GObject`: the row classes that carry the property are not all related
+/// (`AdwEntryRow`'s `use-underline` is its own, `AdwSpinRow`/`AdwComboRow`/`AdwSwitchRow`'s come through
+/// `AdwActionRow`), so a common getter or a cast to one row type would miss half of them -- and passing a
+/// row as a `GtkWidget*` where the type is only forward-declared does not even compile. A class without the
+/// property answers false rather than crashing.
+inline bool row_underlines(GObject* row) {
+  if (row == nullptr) return false;
+  if (g_object_class_find_property(G_OBJECT_GET_CLASS(row), "use-underline") == nullptr) return false;
+  gboolean on = FALSE;
+  g_object_get(row, "use-underline", &on, nullptr);
+  return on != FALSE;
+}
+
 namespace udpscope {
 namespace ui_impl {
 

src/ui_probe.cpp

@@ -1696,6 +1696,12 @@ gboolean prefs_probe(gpointer user) {
   // the `.ui` -- the one mistake neither the build nor `GtkBuilder` sees is a missing id.
   const char* shown =
       a.prefs.dir_row != nullptr ? gtk_editable_get_text(GTK_EDITABLE(a.prefs.dir_row)) : "-";
+  // 4/4: every row's title carries a mnemonic (`_Directory`, `Keep _events`, ...), so a row that does not
+  // ask for `use-underline` draws the underscore instead of underlining the letter.
+  std::fprintf(stderr, "[prefs-probe] mnemonics=%d/4\n",
+               (row_underlines(G_OBJECT(a.prefs.dir_row)) + row_underlines(G_OBJECT(a.prefs.keep_row)) +
+                row_underlines(G_OBJECT(a.prefs.relative_row)) +
+                row_underlines(G_OBJECT(a.prefs.theme_row))));
   std::fprintf(stderr, "[prefs-probe] opened=%d rows=%d dir=%s store=%s\n",
                a.prefs.dialog != nullptr ? 1 : 0, a.prefs.dir_row != nullptr ? 1 : 0,
                a.prefs.dir_row != nullptr ? shown : "-",