[rednafi]

Fixes #2. Stacked on #6.

Change

• The diff has the whole file. A long run of unchanged lines folds into one row: ⋯ 13 unmodified lines @@ -27,7 +27,7 @@. Three lines show beside each change, as before. A run of one line shows, because its fold saves no row. The fold at the end of a file has no @@.
• → on a fold, or a click on it, shows its lines. The cursor goes to the first of them. On a line of code, → still scrolls sideways. The footer shows → show the lines on a fold.
• A fold with a comment on one of its lines always shows, so a comment never hides.
• : with a line in a fold shows the fold and goes to the line.
• A new scope folds the lines again.
• In the split layout, the header goes on across the right side, so the text is not cut.
• The review text for the agent does not change: it skips header rows.

▶   …   …   ⋯ 6 unmodified lines  @@ -7,7 +7,7 @@
    7   7   line 7
   ...
    …   …   ⋯ 13 unmodified lines  @@ -27,7 +27,7 @@
   27  27   line 27
   ...
    …   …   ⋯ 7 unmodified lines

Tests

• diff.TestFolds: the exact rows (line numbers and folds) when no fold, the middle fold, the top and the tail fold, and every fold shows; the header text in both layouts; no line is wider than the screen.
• diff.TestFoldEdges: one line between two margins shows; an insert gives a fold different old and new numbers; a new file has no fold; the split layout has the same folds.
• tui.TestExpandFold: → on a fold shows its 13 lines and moves the cursor to line 14; the footer hint shows only on a fold; → on code scrolls.
• tui.TestClickFold: a click shows the tail fold in the unified layout, the split layout, and with wrap.
• tui.TestFoldKeepsComment: a comment on a line of an expanded fold keeps the fold open after a new scope.
• tui.TestGotoLineInFold: :20 shows the fold of line 20.
• I checked that the tests fail when the fix of each case is removed.
• Checked in Rex with the built binary: →, a click, :20, and the split layout.

Cost

A 20,000-line file with two changes renders in 3.3 ms, up from 3.0 ms.