terminal: cub with reverse mode on left margin

src/terminal/Terminal.zig

@@ -1413,6 +1413,25 @@ pub fn cursorLeft(self: *Terminal, count_req: usize) void {
     else
         self.scrolling_region.left;
 
+    // Handle some edge cases when our cursor is already on the left margin.
+    if (self.screen.cursor.x == left_margin) {
+        switch (wrap_mode) {
+            // In reverse mode, if we're already before the top margin
+            // then we just set our cursor to the top-left and we're done.
+            .reverse => if (self.screen.cursor.y <= top) {
+                self.screen.cursor.x = left_margin;
+                self.screen.cursor.y = top;
+                return;
+            },
+
+            // Handled in while loop
+            .reverse_extended => {},
+
+            // Handled above
+            .none => unreachable,
+        }
+    }
+
     while (true) {
         // We can move at most to the left margin.
         const max = self.screen.cursor.x - left_margin;
@@ -1437,8 +1456,10 @@ pub fn cursorLeft(self: *Terminal, count_req: usize) void {
         }
 
         // If our previous line is not wrapped then we are done.
-        const row = self.screen.getRow(.{ .active = self.screen.cursor.y - 1 });
-        if (wrap_mode != .reverse_extended and !row.isWrapped()) break;
+        if (wrap_mode != .reverse_extended) {
+            const row = self.screen.getRow(.{ .active = self.screen.cursor.y - 1 });
+            if (!row.isWrapped()) break;
+        }
         self.screen.cursor.y -= 1;
         self.screen.cursor.x = right_margin;
         count -= 1;
@@ -5934,6 +5955,24 @@ test "Terminal: cursorLeft reverse wrap with no soft wrap" {
     }
 }
 
+test "Terminal: cursorLeft reverse wrap before left margin" {
+    const alloc = testing.allocator;
+    var t = try init(alloc, 5, 5);
+    defer t.deinit(alloc);
+
+    t.modes.set(.wraparound, true);
+    t.modes.set(.reverse_wrap, true);
+    t.setTopAndBottomMargin(3, 0);
+    t.cursorLeft(1);
+    try t.print('X');
+
+    {
+        var str = try t.plainString(testing.allocator);
+        defer testing.allocator.free(str);
+        try testing.expectEqualStrings("\n\nX", str);
+    }
+}
+
 test "Terminal: cursorLeft extended reverse wrap" {
     const alloc = testing.allocator;
     var t = try init(alloc, 5, 5);

website/app/vt/cub/page.mdx

@@ -12,8 +12,7 @@ or equal to 0, adjust `n` to be 1. If `n` is omitted, `n` defaults to 1.
 This sequence always unsets the pending wrap state.
 
 The leftmost boundary the cursor can move to is determined by the current
-cursor column and the [left margin](#TODO). If the cursor begins to the left
-of the left margin, modify the left margin to be the leftmost column
+cursor column and the [left margin](#TODO). If the cursor begins to the left of the left margin, modify the left margin to be the leftmost column
 for the duration of the sequence. The leftmost column the cursor can be on
 is the left margin.
 
@@ -142,5 +141,22 @@ printf "X"
 |A_________|
 |B_________|
 |_________Xc
-``g
 ```
+
+### CUB V-6: Reverse Wrap Outside of Margins
+
+```bash
+printf "\033[1;1H"
+printf "\033[0J"
+printf "\033[?45h"
+printf "\033[3r"
+printf "\b"
+printf "X"
+```
+
+```
+|__________|
+|__________|
+|Xc________|
+```
+