fix(float): add fixd option

src/nvim/api/keysets.h

@@ -112,6 +112,7 @@ typedef struct {
   String footer_pos;
   String style;
   Boolean noautocmd;
+  Boolean fixed;
 } Dict(float_config);
 
 typedef struct {

src/nvim/api/win_config.c

@@ -160,6 +160,8 @@
 ///   - noautocmd: If true then no buffer-related autocommand events such as
 ///                  |BufEnter|, |BufLeave| or |BufWinEnter| may fire from
 ///                  calling this function.
+///   - fixed: If true when anchor is NW or SW, the float window
+///            would be kept fixed even if the window would be truncated.
 ///
 /// @param[out] err Error details, if any
 ///
@@ -841,6 +843,10 @@ static bool parse_float_config(Dict(float_config) *config, FloatConfig *fconfig,
     fconfig->noautocmd = config->noautocmd;
   }
 
+  if (HAS_KEY_X(config, fixed)) {
+    fconfig->fixed = config->fixed;
+  }
+
   return true;
 #undef HAS_KEY_X
 }

src/nvim/buffer_defs.h

@@ -966,6 +966,7 @@ typedef struct {
   VirtText footer_chunks;
   int footer_width;
   bool noautocmd;
+  bool fixed;
 } FloatConfig;
 
 #define FLOAT_CONFIG_INIT ((FloatConfig){ .height = 0, .width = 0, \
@@ -975,7 +976,8 @@ typedef struct {
                                           .focusable = true, \
                                           .zindex = kZIndexFloatDefault, \
                                           .style = kWinStyleUnused, \
-                                          .noautocmd = false })
+                                          .noautocmd = false, \
+                                          .fixed = false })
 
 // Structure to store last cursor position and topline.  Used by check_lnums()
 // and reset_lnums().

src/nvim/window.c

@@ -1010,6 +1010,10 @@ void ui_ext_win_position(win_T *wp, bool validate)
       comp_col += grid->comp_col;
       comp_row = MAX(MIN(comp_row, Rows - wp->w_height_outer - (p_ch > 0 ? 1 : 0)), 0);
       comp_col = MAX(MIN(comp_col, Columns - wp->w_width_outer), 0);
+      int right_extra = Columns - (int)c.col - wp->w_width - (c.border_chars[2][0] != 0);
+      if (!(c.anchor & kFloatAnchorEast) && c.fixed && right_extra < 0) {
+        comp_col = (int)c.col;
+      }
       wp->w_winrow = comp_row;
       wp->w_wincol = comp_col;
       ui_comp_put_grid(&wp->w_grid_alloc, comp_row, comp_col,