fix(decorations): validate botline for on_win

Problem:
Many decoration providers (treesitter injection highlighting, semantic
token highlighting, inlay hint) rely on the correctness of the `botline`
argument of `on_win` callback. However, `botline` can be smaller than
the actual line number of the last displayed line if some lines are
folded. In such cases, some decorations will be missing in the lines not
covered by `botline`.

Solution:
Validate `botline` when invoking `on_win`.

NOTE:
It seems that the old code was deliberately avoiding this presumably due
to performance reasons. However, I haven't experienced noticeable lag
after this change, and I believe the cost of botline computation would
be much smaller than the cost of decoration providers.

2 files changed, 7 insertions, 9 deletions

diff --git a/src/nvim/api/extmark.c b/src/nvim/api/extmark.c
index 0cf16294b6..e66e25e527 100644
--- a/src/nvim/api/extmark.c
+++ b/src/nvim/api/extmark.c
@@ -1009,10 +1009,8 @@ void nvim_buf_clear_namespace(Buffer buffer, Integer ns_id, Integer line_start,
 ///             - on_buf: called for each buffer being redrawn (before
 ///                 window callbacks)
 ///                 ["buf", bufnr, tick]
-///             - on_win: called when starting to redraw a
-///                 specific window. botline_guess is an approximation
-///                 that does not exceed the last line number.
-///                 ["win", winid, bufnr, topline, botline_guess]
+///             - on_win: called when starting to redraw a specific window.
+///                 ["win", winid, bufnr, topline, botline]
 ///             - on_line: called for each buffer line being redrawn.
 ///                 (The interaction with fold lines is subject to change)
 ///                 ["win", winid, bufnr, row]
diff --git a/src/nvim/decoration_provider.c b/src/nvim/decoration_provider.c
index 6a7ecf102d..9bb499fe4b 100644
--- a/src/nvim/decoration_provider.c
+++ b/src/nvim/decoration_provider.c
@@ -15,6 +15,7 @@
 #include "nvim/log.h"
 #include "nvim/lua/executor.h"
 #include "nvim/message.h"
+#include "nvim/move.h"
 #include "nvim/pos_defs.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
@@ -121,10 +122,9 @@ void decor_providers_invoke_win(win_T *wp)
   // then we would need decor_state.running_decor_provider just like "on_line" below
   assert(kv_size(decor_state.active) == 0);
 
-  linenr_T knownmax = MIN(wp->w_buffer->b_ml.ml_line_count,
-                          ((wp->w_valid & VALID_BOTLINE)
-                           ? wp->w_botline
-                           : MAX(wp->w_topline + wp->w_height_inner, wp->w_botline)));
+  if (kv_size(decor_providers) > 0) {
+    validate_botline(wp);
+  }
 
   for (size_t i = 0; i < kv_size(decor_providers); i++) {
     DecorProvider *p = &kv_A(decor_providers, i);
@@ -138,7 +138,7 @@ void decor_providers_invoke_win(win_T *wp)
       ADD_C(args, BUFFER_OBJ(wp->w_buffer->handle));
       // TODO(bfredl): we are not using this, but should be first drawn line?
       ADD_C(args, INTEGER_OBJ(wp->w_topline - 1));
-      ADD_C(args, INTEGER_OBJ(knownmax - 1));
+      ADD_C(args, INTEGER_OBJ(wp->w_botline - 1 - 1));
       if (!decor_provider_invoke((int)i, "win", p->redraw_win, args, true)) {
         kv_A(decor_providers, i).state = kDecorProviderWinDisabled;
       }