feature: Improve handling headings

# Details

Previously 2 problems could occur with headings:

1. If the icon text was too long it would overwrite content
2. Concealed text was not accounted for exacerbating problem 1

To fix both of these use the width of concealed text when calculating padding.

If the padding is negative (not enough space) then use an inline
extmark and conceal the underlying '#'s, this will only happen
when using neovim >= 0.10.0. Negative padding on older versions
will skip rendering rather than overwriting text.

Other change included refactors unit tests to use common mark detail
creators in util, reduces the amount of boilerplate.

Author: MeanderingProgrammer

Diff for: doc/limitations.md

@@ -4,7 +4,9 @@
 
 [ISSUE #35](https://github.com/MeanderingProgrammer/markdown.nvim/issues/35)
 
-Text that extends beyond available space will overwrite content.
+Text that extends beyond available space will can overwrite content.
+
+This has been fixed for headings when using neovim >= `0.10.0`.
 
 ## `LaTeX` Formula Positioning
 

Diff for: lua/render-markdown/handler/markdown.lua

@@ -41,18 +41,38 @@ M.render_node = function(namespace, buf, capture, node)
         local background = list.clamp(heading.backgrounds, level)
         local foreground = list.clamp(heading.foregrounds, level)
 
-        -- Available width is level + 1, where level = number of `#` characters and one is
-        -- added to account for the space after the last `#` but before the heading title
-        local padding = level + 1 - str.width(icon)
+        -- Available width is level + 1 - concealed, where level = number of `#` characters, one
+        -- is added to account for the space after the last `#` but before the heading title,
+        -- and concealed text is subtracted since that space is not usable
+        local padding = level + 1 - ts.concealed(buf, info) - str.width(icon)
 
-        vim.api.nvim_buf_set_extmark(buf, namespace, info.start_row, 0, {
-            end_row = info.end_row + 1,
-            end_col = 0,
-            hl_group = background,
-            virt_text = { { str.pad(icon, padding), { foreground, background } } },
-            virt_text_pos = 'overlay',
-            hl_eol = true,
-        })
+        if padding < 0 then
+            -- Requires inline extmarks to place when there is not enough space available
+            if util.has_10 then
+                vim.api.nvim_buf_set_extmark(buf, namespace, info.start_row, info.start_col, {
+                    end_row = info.end_row,
+                    end_col = info.end_col,
+                    virt_text = { { icon, { foreground, background } } },
+                    virt_text_pos = 'inline',
+                    conceal = '',
+                })
+                vim.api.nvim_buf_set_extmark(buf, namespace, info.start_row, 0, {
+                    end_row = info.end_row + 1,
+                    end_col = 0,
+                    hl_group = background,
+                    hl_eol = true,
+                })
+            end
+        else
+            vim.api.nvim_buf_set_extmark(buf, namespace, info.start_row, 0, {
+                end_row = info.end_row + 1,
+                end_col = 0,
+                hl_group = background,
+                virt_text = { { str.pad(icon, padding), { foreground, background } } },
+                virt_text_pos = 'overlay',
+                hl_eol = true,
+            })
+        end
 
         M.render_sign(namespace, buf, info, list.cycle(heading.signs, level), foreground)
     elseif capture == 'dash' then

Diff for: tests/box_dash_quote_spec.lua

@@ -11,48 +11,9 @@ async_tests.describe('box_dash_quote.md', function()
         vim.list_extend(expected, util.heading(0, 1))
 
         -- Checkboxes
-        vim.list_extend(expected, {
-            -- Unchecked, conceal list marker
-            {
-                row = { 2, 2 },
-                col = { 0, 2 },
-                conceal = '',
-            },
-            -- Unchecked, checkbox
-            {
-                row = { 2, 2 },
-                col = { 2, 5 },
-                virt_text = { { ' 󰄱 ', '@markup.list.unchecked' } },
-                virt_text_pos = 'overlay',
-            },
-            -- Checked, conceal list marker
-            {
-                row = { 3, 3 },
-                col = { 0, 2 },
-                conceal = '',
-            },
-            -- Checked, checkbox
-            {
-                row = { 3, 3 },
-                col = { 2, 5 },
-                virt_text = { { ' 󰱒 ', '@markup.heading' } },
-                virt_text_pos = 'overlay',
-            },
-            -- Custom todo, conceal list marker
-            {
-                row = { 4, 4 },
-                col = { 0, 2 },
-                conceal = '',
-            },
-            -- Custom todo, checkbox
-            {
-                row = { 4, 4 },
-                col = { 2, 5 },
-                virt_text = { { ' 󰥔 ', '@markup.raw' } },
-                virt_text_pos = 'inline',
-                conceal = '',
-            },
-        })
+        vim.list_extend(expected, util.checkbox(2, ' 󰄱 ', '@markup.list.unchecked', false))
+        vim.list_extend(expected, util.checkbox(3, ' 󰱒 ', '@markup.heading', false))
+        vim.list_extend(expected, util.checkbox(4, ' 󰥔 ', '@markup.raw', true))
 
         -- Line break
         vim.list_extend(expected, {
@@ -66,18 +27,8 @@ async_tests.describe('box_dash_quote.md', function()
 
         -- Quote lines
         vim.list_extend(expected, {
-            {
-                row = { 8, 8 },
-                col = { 0, 4 },
-                virt_text = { { '  ▋ ', '@markup.quote' } },
-                virt_text_pos = 'overlay',
-            },
-            {
-                row = { 9, 9 },
-                col = { 0, 4 },
-                virt_text = { { '  ▋ ', '@markup.quote' } },
-                virt_text_pos = 'overlay',
-            },
+            util.quote(8, '  %s ', '@markup.quote'),
+            util.quote(9, '  %s ', '@markup.quote'),
         })
 
         local actual = util.get_actual_marks()