feat(extmarks): virtual text can be right-aligned, truncated #31921

Problem: Right aligned virtual text can cover up buffer text if virtual
text is too long

Solution: An additional option for `virt_text_pos` called
`eol_right_align` has been added to truncate virtual text if it would
have otherwise covered up buffer text. This ensures the virtual text
extends no further left than EOL.

4 files changed, 58 insertions, 6 deletions

diff --git a/src/nvim/api/extmark.c b/src/nvim/api/extmark.c
index e66140da5a..778e857057 100644
--- a/src/nvim/api/extmark.c
+++ b/src/nvim/api/extmark.c
@@ -400,6 +400,15 @@ Array nvim_buf_get_extmarks(Buffer buffer, Integer ns_id, Object start, Object e
 ///                   (highest priority last).
 ///               - virt_text_pos : position of virtual text. Possible values:
 ///                 - "eol": right after eol character (default).
+///                 - "eol_right_align": display right aligned in the window
+///                                      unless the virtual text is longer than
+///                                      the space available. If the virtual
+///                                      text is too long, it is truncated to
+///                                      fit in the window after the EOL
+///                                      character. If the line is wrapped, the
+///                                      virtual text is shown after the end of
+///                                      the line rather than the previous
+///                                      screen line.
 ///                 - "overlay": display over the specified column, without
 ///                              shifting the underlying text.
 ///                 - "right_align": display right aligned in the window.
@@ -620,6 +629,8 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
       virt_text.pos = kVPosOverlay;
     } else if (strequal("right_align", str.data)) {
       virt_text.pos = kVPosRightAlign;
+    } else if (strequal("eol_right_align", str.data)) {
+      virt_text.pos = kVPosEndOfLineRightAlign;
     } else if (strequal("inline", str.data)) {
       virt_text.pos = kVPosInline;
     } else {
diff --git a/src/nvim/decoration.h b/src/nvim/decoration.h
index a2f4fefd45..bdbb1795cb 100644
--- a/src/nvim/decoration.h
+++ b/src/nvim/decoration.h
@@ -15,8 +15,8 @@
 // actual Decor* data is in decoration_defs.h
 
 /// Keep in sync with VirtTextPos in decoration_defs.h
-EXTERN const char *const virt_text_pos_str[]
-INIT( = { "eol", "overlay", "win_col", "right_align", "inline" });
+EXTERN const char *const virt_text_pos_str[] INIT( = { "eol", "eol_right_align", "inline",
+                                                       "overlay", "right_align", "win_col" });
 
 /// Keep in sync with HlMode in decoration_defs.h
 EXTERN const char *const hl_mode_str[] INIT( = { "", "replace", "combine", "blend" });
diff --git a/src/nvim/decoration_defs.h b/src/nvim/decoration_defs.h
index 58ba93a7ba..36ad6df7a0 100644
--- a/src/nvim/decoration_defs.h
+++ b/src/nvim/decoration_defs.h
@@ -19,10 +19,11 @@ typedef kvec_t(VirtTextChunk) VirtText;
 /// Keep in sync with virt_text_pos_str[] in decoration.h
 typedef enum {
   kVPosEndOfLine,
+  kVPosEndOfLineRightAlign,
+  kVPosInline,
   kVPosOverlay,
-  kVPosWinCol,
   kVPosRightAlign,
-  kVPosInline,
+  kVPosWinCol,
 } VirtTextPos;
 
 typedef kvec_t(struct virt_line { VirtText line; bool left_col; }) VirtLines;
diff --git a/src/nvim/drawline.c b/src/nvim/drawline.c
index 4a7bc9170a..37a42917b0 100644
--- a/src/nvim/drawline.c
+++ b/src/nvim/drawline.c
@@ -263,6 +263,9 @@ static void draw_virt_text(win_T *wp, buf_T *buf, int col_off, int *end_col, int
   int *const indices = state->ranges_i.items;
   DecorRangeSlot *const slots = state->slots.items;
 
+  /// Total width of all virtual text with "eol_right_align" alignment
+  int totalWidthOfEolRightAlignedVirtText = 0;
+
   for (int i = 0; i < end; i++) {
     DecorRange *item = &slots[indices[i]].range;
     if (!(item->start_row == state->row && decor_virt_pos(item))) {
@@ -277,7 +280,44 @@ static void draw_virt_text(win_T *wp, buf_T *buf, int col_off, int *end_col, int
     if (decor_virt_pos(item) && item->draw_col == -1) {
       bool updated = true;
       VirtTextPos pos = decor_virt_pos_kind(item);
-      if (pos == kVPosRightAlign) {
+
+      if (do_eol && pos == kVPosEndOfLineRightAlign) {
+        int eolOffset = 0;
+        if (totalWidthOfEolRightAlignedVirtText == 0) {
+          // Look ahead to the remaining decor items
+          for (int j = i; j < end; j++) {
+            /// A future decor to be handled in this function's call
+            DecorRange *lookaheadItem = &slots[indices[j]].range;
+
+            if (lookaheadItem->start_row != state->row
+                || !decor_virt_pos(lookaheadItem)
+                || lookaheadItem->draw_col != -1) {
+              continue;
+            }
+
+            /// The Virtual Text of the decor item we're looking ahead to
+            DecorVirtText *lookaheadVt = NULL;
+            if (item->kind == kDecorKindVirtText) {
+              assert(item->data.vt);
+              lookaheadVt = item->data.vt;
+            }
+
+            if (decor_virt_pos_kind(lookaheadItem) == kVPosEndOfLineRightAlign) {
+              // An extra space is added for single character spacing in EOL alignment
+              totalWidthOfEolRightAlignedVirtText += (lookaheadVt->width + 1);
+            }
+          }
+
+          // Remove one space from the total width since there's no single space after the last entry
+          totalWidthOfEolRightAlignedVirtText--;
+
+          if (totalWidthOfEolRightAlignedVirtText <= (right_pos - state->eol_col)) {
+            eolOffset = right_pos - totalWidthOfEolRightAlignedVirtText - state->eol_col;
+          }
+        }
+
+        item->draw_col = state->eol_col + eolOffset;
+      } else if (pos == kVPosRightAlign) {
         right_pos -= vt->width;
         item->draw_col = right_pos;
       } else if (pos == kVPosEndOfLine && do_eol) {
@@ -304,7 +344,7 @@ static void draw_virt_text(win_T *wp, buf_T *buf, int col_off, int *end_col, int
       int vcol = item->draw_col - col_off;
       int col = draw_virt_text_item(buf, item->draw_col, vt->data.virt_text,
                                     vt->hl_mode, max_col, vcol);
-      if (vt->pos == kVPosEndOfLine && do_eol) {
+      if (do_eol && ((vt->pos == kVPosEndOfLine) || (vt->pos == kVPosEndOfLineRightAlign))) {
         state->eol_col = col + 1;
       }
       *end_col = MAX(*end_col, col);