gdb/tui: add 'set tui mouse-events off' to restore mouse selection

Rationale:
I use the mouse with my terminal to select and copy text. In gdb, I use
the mouse to select a function name to set a breakpoint, or a variable
name to print, for example.

When gdb is compiled with ncurses mouse support, gdb's TUI mode
intercepts mouse events. Left-clicking and dragging, which would
normally select text, seems to do nothing. This means I cannot select
text using my mouse anymore. This makes it harder to set breakpoints,
print variables, etc.

Solution:
I tried to fix this issue by editing the 'mousemask' call to only enable
buttons 4 and 5. However, this still caused my terminal (gnome-terminal)
to not allow text to be selected. The only way I could make it work is
by calling 'mousemask (0, NULL);'. But doing so disables the mouse code
entirely, which other people might want.

I therefore decided to make a setting in gdb called 'tui mouse-events'.
If enabled (the default), the behavior is as it is now: terminal mouse
events are given to gdb, disabling the terminal's default behavior.
If disabled (opt-in), the behavior is as it was before the year 2020:
terminal mouse events are not given to gdb, therefore the mouse can be
used to select and copy text.

Notes:
I am not attached to the setting name or its description. Feel free to
suggest better wording.

Testing:
I tested this change in gnome-terminal by performing the following steps
manually:

1. Run: gdb --args ./myprogram
2. Enable TUI: press ctrl-x ctrl-a
3. Click and drag text with the mouse. Observe no selection.
4. Input: set tui mouse-events off
5. Click and drag text with the mouse. Observe that selection works now.
6. Input: set tui mouse-events on.
7. Click and drag text with the mouse. Observe no selection.

3 files changed, 27 insertions, 1 deletions

diff --git a/gdb/tui/tui-io.c b/gdb/tui/tui-io.c
index 8cb68d1..b8954af 100644
--- a/gdb/tui/tui-io.c
+++ b/gdb/tui/tui-io.c
@@ -657,7 +657,8 @@ static void
 tui_prep_terminal (int notused1)
 {
 #ifdef NCURSES_MOUSE_VERSION
-  mousemask (ALL_MOUSE_EVENTS, NULL);
+  if (tui_enable_mouse)
+    mousemask (ALL_MOUSE_EVENTS, NULL);
 #endif
 }
 
diff --git a/gdb/tui/tui-win.c b/gdb/tui/tui-win.c
index 93cf45e..a0a1233 100644
--- a/gdb/tui/tui-win.c
+++ b/gdb/tui/tui-win.c
@@ -872,6 +872,17 @@ tui_show_compact_source (struct ui_file *file, int from_tty,
   gdb_printf (file, _("TUI source window compactness is %s.\n"), value);
 }
 
+bool tui_enable_mouse = true;
+
+/* Implement 'show tui mouse-events'.  */
+
+static void
+show_tui_mouse_events (struct ui_file *file, int from_tty,
+		       struct cmd_list_element *c, const char *value)
+{
+  gdb_printf (file, _("TUI mouse events are %s.\n"), value);
+}
+
 /* Set the tab width of the specified window.  */
 static void
 tui_set_tab_width_command (const char *arg, int from_tty)
@@ -1254,6 +1265,17 @@ in a compact form.  The compact form uses less horizontal space."),
 			   tui_set_compact_source, tui_show_compact_source,
 			   &tui_setlist, &tui_showlist);
 
+  add_setshow_boolean_cmd ("mouse-events", class_tui,
+			   &tui_enable_mouse, _("\
+Set whether TUI mode handles mouse clicks."), _("\
+Show whether TUI mode handles mouse clicks."), _("\
+When on (default), mouse clicks control the TUI and can be accessed by Python\n\
+extensions.  When off, mouse clicks are handled by the terminal, enabling\n\
+terminal-native text selection."),
+			   nullptr,
+			   show_tui_mouse_events,
+			   &tui_setlist, &tui_showlist);
+
   add_setshow_boolean_cmd ("tui-current-position", class_maintenance,
 			   &style_tui_current_position, _("\
 Set whether to style text highlighted by the TUI's current position indicator."),
diff --git a/gdb/tui/tui-win.h b/gdb/tui/tui-win.h
index 3d35f1d..c0a7732 100644
--- a/gdb/tui/tui-win.h
+++ b/gdb/tui/tui-win.h
@@ -51,6 +51,9 @@ struct cmd_list_element **tui_get_cmd_list (void);
 /* Whether compact source display should be used.  */
 extern bool compact_source;
 
+/* Whether the TUI should intercept terminal mouse events.  */
+extern bool tui_enable_mouse;
+
 /* Whether to style the source and assembly code highlighted by the TUI's
    current position indicator.  */
 extern bool style_tui_current_position;