feat(api): add forward and back mouse buttons

runtime/doc/api.txt

@@ -1145,7 +1145,7 @@ nvim_input_mouse({button}, {action}, {modifier}, {grid}, {row}, {col})
 
     Parameters: ~
       • {button}    Mouse button: one of "left", "right", "middle", "wheel",
-                    "move".
+                    "move", "x1", "x2".
       • {action}    For ordinary buttons, one of "press", "drag", "release".
                     For the wheel, one of "up", "down", "left", "right".
                     Ignored for "move".

runtime/doc/news.txt

@@ -268,6 +268,8 @@ The following new APIs and features were added.
 • 'completeopt' option supports "popup" flags to show extra information in
   in floating window.
 
+• Added `x1` and `x2` mouse buttons as possible arguments to |nvim_input_mouse()|
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
 

runtime/lua/vim/_meta/api.lua

@@ -1375,7 +1375,7 @@ function vim.api.nvim_input(keys) end
 --- processed soon by the event loop.
 ---
 --- @param button string Mouse button: one of "left", "right", "middle", "wheel",
----                 "move".
+---                 "move", "x1", "x2".
 --- @param action string For ordinary buttons, one of "press", "drag", "release".
 ---                 For the wheel, one of "up", "down", "left", "right".
 ---                 Ignored for "move".

src/nvim/api/vim.c

@@ -345,7 +345,8 @@ Integer nvim_input(String keys)
 ///       mouse input in a GUI. The deprecated pseudokey form
 ///       ("<LeftMouse><col,row>") of |nvim_input()| has the same limitation.
 ///
-/// @param button Mouse button: one of "left", "right", "middle", "wheel", "move".
+/// @param button Mouse button: one of "left", "right", "middle", "wheel", "move",
+///               "x1", "x2".
 /// @param action For ordinary buttons, one of "press", "drag", "release".
 ///               For the wheel, one of "up", "down", "left", "right". Ignored for "move".
 /// @param modifier String of modifiers each represented by a single char.
@@ -376,6 +377,10 @@ void nvim_input_mouse(String button, String action, String modifier, Integer gri
     code = KE_RIGHTMOUSE;
   } else if (strequal(button.data, "wheel")) {
     code = KE_MOUSEDOWN;
+  } else if (strequal(button.data, "x1")) {
+    code = KE_X1MOUSE;
+  } else if (strequal(button.data, "x2")) {
+    code = KE_X2MOUSE;
   } else if (strequal(button.data, "move")) {
     code = KE_MOUSEMOVE;
   } else {

src/nvim/os/input.c

@@ -311,7 +311,8 @@ static uint8_t check_multiclick(int code, int grid, int row, int col)
   }
 
   // For click events the number of clicks is updated.
-  if (code == KE_LEFTMOUSE || code == KE_RIGHTMOUSE || code == KE_MIDDLEMOUSE) {
+  if (code == KE_LEFTMOUSE || code == KE_RIGHTMOUSE || code == KE_MIDDLEMOUSE
+      || code == KE_X1MOUSE || code == KE_X2MOUSE) {
     uint64_t mouse_time = os_hrtime();    // time of current mouse click (ns)
     // compute the time elapsed since the previous mouse click and
     // convert p_mouse from ms to ns

test/functional/ui/mouse_spec.lua

@@ -758,6 +758,25 @@ describe('ui/mouse/input', function()
     feed('<cr>')
   end)
 
+  it('x1 and x2 can be triggered by api', function()
+    meths.set_var('x1_pressed', 0)
+    meths.set_var('x1_released', 0)
+    meths.set_var('x2_pressed', 0)
+    meths.set_var('x2_released', 0)
+    command('nnoremap <X1Mouse> <Cmd>let g:x1_pressed += 1<CR>')
+    command('nnoremap <X1Release> <Cmd>let g:x1_released += 1<CR>')
+    command('nnoremap <X2Mouse> <Cmd>let g:x2_pressed += 1<CR>')
+    command('nnoremap <X2Release> <Cmd>let g:x2_released += 1<CR>')
+    meths.input_mouse('x1', 'press', '', 0, 0, 0)
+    meths.input_mouse('x1', 'release', '', 0, 0, 0)
+    meths.input_mouse('x2', 'press', '', 0, 0, 0)
+    meths.input_mouse('x2', 'release', '', 0, 0, 0)
+    eq(1, meths.get_var('x1_pressed'), 'x1 pressed once')
+    eq(1, meths.get_var('x1_released'), 'x1 released once')
+    eq(1, meths.get_var('x2_pressed'), 'x2 pressed once')
+    eq(1, meths.get_var('x2_released'), 'x2 released once')
+  end)
+
   it('dragging vertical separator', function()
     screen:try_resize(45, 5)
     command('setlocal nowrap')