OK, nothing wrong with that.  You got rid of those #if 0 items too, that's good.

Yes, it does.  After the patch, here is the debug info showing a valid m_custom_style from the start rather than the random numbers:

M_ACTIVE_DOCK:0x167c240  DOCK: 0x1881f50
m_custom_style:1  w_old:0x167c240  w_new:0x1881f50  this:0x167c240
m_custom_style:1  w_old:0x167c240  w_new:0x1881f50  this:0x1881f50
m_custom_style:1  w_old:0x167c240  w_new:0x1881f50  this:0x191b130
m_custom_style:1  w_old:0x167c240  w_new:0x1881f50  this:0x19b63e0
m_custom_style:1  w_old:0x167c240  w_new:0x1881f50  this:0x1b59e10
m_custom_style:1  w_old:0x167c240  w_new:0x1881f50  this:0x1f80b10
m_custom_style:1  w_old:0x167c240  w_new:0x1881f50  this:0x1c86110
M_ACTIVE_DOCK:0x1881f50  DOCK: 0x19b63e0
m_custom_style:1  w_old:0x1881f50  w_new:0x19b63e0  this:0x167c240
m_custom_style:1  w_old:0x1881f50  w_new:0x19b63e0  this:0x1881f50
m_custom_style:1  w_old:0x1881f50  w_new:0x19b63e0  this:0x191b130
m_custom_style:1  w_old:0x1881f50  w_new:0x19b63e0  this:0x19b63e0
m_custom_style:1  w_old:0x1881f50  w_new:0x19b63e0  this:0x1b59e10
[etc]

I'm attaching a diff file of the annotation in case you need to test something again.

(file #44101)

As part of fix for Bug #53276 I originally made a change that basically reversed the order in which the GUI layout and GUI settings are done at startup.  That is, I made the change:

@@ -1203,34 +1188,35 @@ namespace octave
   }

   void main_window::read_settings (void)
   {
     QSettings *settings = resource_manager::get_settings ();

     if (! settings)
       {
         qDebug ("Error: QSettings pointer from resource manager is NULL.");
         return;
       }

+    emit settings_changed (settings);
+
     set_window_layout (settings);

     // restore the list of the last directories
     QStringList curr_dirs
       = settings->value ("MainWindow/current_directory_list").toStringList ();
     for (int i=0; i < curr_dirs.size (); i++)
       {
         m_current_directory_combo_box->addItem (curr_dirs.at (i));
       }
-    emit settings_changed (settings);
   }

The reason I moved the emitted settings_changed() sooner is that it looked to me that restoring the layout was emitting settings_changed() which in turn called slots:

  void
  octave_dock_widget::handle_active_dock_changed (octave_dock_widget *w_old,
                                                  octave_dock_widget *w_new)
  {
    if (m_custom_style && this == w_old)
      {
        set_style (false);
        update ();
      }

    if (m_custom_style && this == w_new)
      {
        set_style (true);
        update ();
      }
  }

and m_custom_style and all the similar variables within set_style() had not been initialized yet.  To see that this is the case, here's a dump of what's happening, in which ALL-CAPS is what's happening inside main_window::focus_changed() and lower case is what's happening inside octave_dock_widget::handle_active_dock_changed():

M_ACTIVE_DOCK:0  DOCK:0xfaa5a0
m_custom_style:1  w_old:0  w_new:0xfaa5a0  this:0xd8bb10
m_custom_style:10  w_old:0  w_new:0xfaa5a0  this:0xf702b0
m_custom_style:41  w_old:0  w_new:0xfaa5a0  this:0xfaa5a0
m_custom_style:0  w_old:0  w_new:0xfaa5a0  this:0x10ffd90
m_custom_style:115  w_old:0  w_new:0xfaa5a0  this:0x1445850
m_custom_style:107  w_old:0  w_new:0xfaa5a0  this:0x141e220
m_custom_style:47  w_old:0  w_new:0xfaa5a0  this:0x14a38b0
M_ACTIVE_DOCK:0xfaa5a0  DOCK:0xd8bb10
m_custom_style:1  w_old:0xfaa5a0  w_new:0xd8bb10  this:0xd8bb10
m_custom_style:10  w_old:0xfaa5a0  w_new:0xd8bb10  this:0xf702b0
m_custom_style:41  w_old:0xfaa5a0  w_new:0xd8bb10  this:0xfaa5a0
m_custom_style:0  w_old:0xfaa5a0  w_new:0xd8bb10  this:0x10ffd90
m_custom_style:115  w_old:0xfaa5a0  w_new:0xd8bb10  this:0x1445850
m_custom_style:107  w_old:0xfaa5a0  w_new:0xd8bb10  this:0x141e220
m_custom_style:47  w_old:0xfaa5a0  w_new:0xd8bb10  this:0x14a38b0
M_ACTIVE_DOCK:0xd8bb10  DOCK:0xf702b0
m_custom_style:1  w_old:0xd8bb10  w_new:0xf702b0  this:0xd8bb10
m_custom_style:10  w_old:0xd8bb10  w_new:0xf702b0  this:0xf702b0
m_custom_style:41  w_old:0xd8bb10  w_new:0xf702b0  this:0xfaa5a0
m_custom_style:0  w_old:0xd8bb10  w_new:0xf702b0  this:0x10ffd90
m_custom_style:115  w_old:0xd8bb10  w_new:0xf702b0  this:0x1445850
m_custom_style:107  w_old:0xd8bb10  w_new:0xf702b0  this:0x141e220
m_custom_style:47  w_old:0xd8bb10  w_new:0xf702b0  this:0x14a38b0
M_ACTIVE_DOCK:0xf702b0  DOCK:0x10ffd90
m_custom_style:1  w_old:0xf702b0  w_new:0x10ffd90  this:0xd8bb10
m_custom_style:10  w_old:0xf702b0  w_new:0x10ffd90  this:0xf702b0
m_custom_style:41  w_old:0xf702b0  w_new:0x10ffd90  this:0xfaa5a0
m_custom_style:0  w_old:0xf702b0  w_new:0x10ffd90  this:0x10ffd90
m_custom_style:115  w_old:0xf702b0  w_new:0x10ffd90  this:0x1445850
m_custom_style:107  w_old:0xf702b0  w_new:0x10ffd90  this:0x141e220
m_custom_style:47  w_old:0xf702b0  w_new:0x10ffd90  this:0x14a38b0
M_ACTIVE_DOCK:0x10ffd90  DOCK:0x1445850
m_custom_style:1  w_old:0x10ffd90  w_new:0x1445850  this:0xd8bb10
m_custom_style:10  w_old:0x10ffd90  w_new:0x1445850  this:0xf702b0
m_custom_style:41  w_old:0x10ffd90  w_new:0x1445850  this:0xfaa5a0
m_custom_style:0  w_old:0x10ffd90  w_new:0x1445850  this:0x10ffd90
m_custom_style:115  w_old:0x10ffd90  w_new:0x1445850  this:0x1445850
m_custom_style:107  w_old:0x10ffd90  w_new:0x1445850  this:0x141e220
m_custom_style:47  w_old:0x10ffd90  w_new:0x1445850  this:0x14a38b0
M_ACTIVE_DOCK:0x1445850  DOCK:0x14a38b0
m_custom_style:1  w_old:0x1445850  w_new:0x14a38b0  this:0xd8bb10
m_custom_style:10  w_old:0x1445850  w_new:0x14a38b0  this:0xf702b0
m_custom_style:41  w_old:0x1445850  w_new:0x14a38b0  this:0xfaa5a0
m_custom_style:0  w_old:0x1445850  w_new:0x14a38b0  this:0x10ffd90
m_custom_style:115  w_old:0x1445850  w_new:0x14a38b0  this:0x1445850
m_custom_style:107  w_old:0x1445850  w_new:0x14a38b0  this:0x141e220
m_custom_style:47  w_old:0x1445850  w_new:0x14a38b0  this:0x14a38b0
M_ACTIVE_DOCK:0x14a38b0  DOCK:0x141e220
m_custom_style:1  w_old:0x14a38b0  w_new:0x141e220  this:0xd8bb10
m_custom_style:10  w_old:0x14a38b0  w_new:0x141e220  this:0xf702b0
m_custom_style:41  w_old:0x14a38b0  w_new:0x141e220  this:0xfaa5a0
m_custom_style:0  w_old:0x14a38b0  w_new:0x141e220  this:0x10ffd90
m_custom_style:115  w_old:0x14a38b0  w_new:0x141e220  this:0x1445850
m_custom_style:107  w_old:0x14a38b0  w_new:0x141e220  this:0x141e220
m_custom_style:47  w_old:0x14a38b0  w_new:0x141e220  this:0x14a38b0
m_custom_style:1  w_old:0  w_new:0x141e220  this:0xd8bb10
m_custom_style:1  w_old:0  w_new:0x141e220  this:0xf702b0
m_custom_style:1  w_old:0  w_new:0x141e220  this:0xfaa5a0
m_custom_style:1  w_old:0  w_new:0x141e220  this:0x10ffd90
m_custom_style:1  w_old:0  w_new:0x141e220  this:0x1445850
m_custom_style:1  w_old:0  w_new:0x141e220  this:0x141e220
m_custom_style:1  w_old:0  w_new:0x141e220  this:0x14a38b0
M_ACTIVE_DOCK:0x141e220  DOCK:0xd8bb10
m_custom_style:1  w_old:0x141e220  w_new:0xd8bb10  this:0xd8bb10
m_custom_style:1  w_old:0x141e220  w_new:0xd8bb10  this:0xf702b0
m_custom_style:1  w_old:0x141e220  w_new:0xd8bb10  this:0xfaa5a0
m_custom_style:1  w_old:0x141e220  w_new:0xd8bb10  this:0x10ffd90
m_custom_style:1  w_old:0x141e220  w_new:0xd8bb10  this:0x1445850
m_custom_style:1  w_old:0x141e220  w_new:0xd8bb10  this:0x141e220
m_custom_style:1  w_old:0x141e220  w_new:0xd8bb10  this:0x14a38b0
>> M_ACTIVE_DOCK:0xd8bb10  DOCK:0xfaa5a0
m_custom_style:1  w_old:0xd8bb10  w_new:0xfaa5a0  this:0xd8bb10
m_custom_style:1  w_old:0xd8bb10  w_new:0xfaa5a0  this:0xf702b0
m_custom_style:1  w_old:0xd8bb10  w_new:0xfaa5a0  this:0xfaa5a0
m_custom_style:1  w_old:0xd8bb10  w_new:0xfaa5a0  this:0x10ffd90
m_custom_style:1  w_old:0xd8bb10  w_new:0xfaa5a0  this:0x1445850
m_custom_style:1  w_old:0xd8bb10  w_new:0xfaa5a0  this:0x141e220
m_custom_style:1  w_old:0xd8bb10  w_new:0xfaa5a0  this:0x14a38b0
>> M_ACTIVE_DOCK:0xfaa5a0  DOCK:0xd8bb10
m_custom_style:1  w_old:0xfaa5a0  w_new:0xd8bb10  this:0xd8bb10
m_custom_style:1  w_old:0xfaa5a0  w_new:0xd8bb10  this:0xf702b0
m_custom_style:1  w_old:0xfaa5a0  w_new:0xd8bb10  this:0xfaa5a0
m_custom_style:1  w_old:0xfaa5a0  w_new:0xd8bb10  this:0x10ffd90
m_custom_style:1  w_old:0xfaa5a0  w_new:0xd8bb10  this:0x1445850
m_custom_style:1  w_old:0xfaa5a0  w_new:0xd8bb10  this:0x141e220
m_custom_style:1  w_old:0xfaa5a0  w_new:0xd8bb10  this:0x14a38b0

Notice how m_custome_style is some random value for all the octave_dock_widgets in the first pass.  Then after the "emit settings_changed (settings)" the m_custom_style has been initialized.  The issue with the change I made (which ostensibly initializes the variables first) is that not having the octave_dock_widget::handle_settings() call after the layout meant the File Browser was left highlighted when it shouldn't have been (until activating and deactivating corrected the matter).  That because at the end of octave_dock_widget::handle_settings() is the following:

#endif

    notice_settings (settings);  // call individual handler

    set_style (false);
  }

That is, the set_style(false) resets the octave_dock_widget highlight state to off.  Whether this should be done is one question, but it looks like currently some other code is doing a re-activate of m_active_dock after a "Preferences...", so that is covered.

In summary, the issue is mainly that m_custom_style, m_bg_color_active, m_fg_color_active, m_icon_color_active, m_bg_color, m_fg_color, and m_icon_color are being used via main_window::read_settings() before they are initialized to anything.  I suppose the worst consequence of this is that the octave_dock_widgets are (non-visibly) being assigned random colors that are then overwritten just prior to making the whole application visible.  In other words, this doesn't appear to be a segfault type of issue.  One approach would be to issue

+    emit settings_changed (settings);

both before and after the a layout code (the first initializes the m_custom_style, etc. and the second clears all the highlights).  However, probably the more proper thing to do is to initialize the m_custom_style, etc. in the octave_dock_widget::octave_dock_widget() constructor.