Set terminal alternate buffer height as terminal height.

https://www.xfree86.org/4.8.0/ctlseqs.html#The%20Alternate%20Screen%20Buffer

src/terminal/buffer.c

@@ -137,6 +137,61 @@ guac_terminal_buffer* guac_terminal_buffer_alloc(int rows,
 
 }
 
+guac_terminal_buffer* guac_terminal_buffer_realloc(guac_terminal_buffer* buffer, int rows) {
+
+    int i;
+    guac_terminal_buffer_row* row;
+
+    /* No change */
+    if (rows == buffer->available)
+        return buffer;
+
+    /* Too many rows */
+    if (rows < buffer->available) {
+
+        /* Last buffer row */
+        row = buffer->rows + buffer->available;
+
+        /* Free excess rows characters */
+        for (i=rows; i>buffer->available; i++) {
+
+            /* Free row characters */
+            guac_mem_free(row->characters);
+
+            /* Previous row */
+            row++;
+        }
+    }
+
+    /* Realloc buffer rows */
+    buffer->rows = guac_mem_realloc(buffer->rows, sizeof(guac_terminal_buffer_row) * rows);
+
+    /* Init new rows characters */
+    if (rows > buffer->available) {
+
+        /* First uninitialized buffer row */
+        row = buffer->rows + buffer->available;
+
+        /* Init new scrollback rows */
+        for (i=buffer->available; i<rows; i++) {
+
+            /* Allocate row  */
+            row->available = GUAC_TERMINAL_BUFFER_ROW_MIN_SIZE;
+            row->length = 0;
+            row->wrapped_row = false;
+            row->characters = guac_mem_alloc(sizeof(guac_terminal_char), row->available);
+
+            /* Next row */
+            row++;
+        }
+    }
+
+    /* Update scrollback length */
+    buffer->available = rows;
+
+    return buffer;
+}
+
 void guac_terminal_buffer_free(guac_terminal_buffer* buffer) {
 
     int i;

src/terminal/terminal.c

@@ -395,10 +395,6 @@ guac_terminal* guac_terminal_create(guac_client* client,
     if (initial_scrollback < GUAC_TERMINAL_MAX_ROWS)
         initial_scrollback = GUAC_TERMINAL_MAX_ROWS;
 
-    /* Init current and alternate buffer */
-    term->current_buffer = term->normal_buffer = guac_terminal_buffer_alloc(initial_scrollback, &default_char);
-    term->alternate_buffer = guac_terminal_buffer_alloc(GUAC_TERMINAL_MAX_ROWS, &default_char);
-
     /* Init display */
     term->display = guac_terminal_display_alloc(client,
             options->font_name, options->font_size, options->dpi,
@@ -444,6 +440,10 @@ guac_terminal* guac_terminal_create(guac_client* client,
     term->height = adjusted_height;
     term->width = adjusted_width;
 
+    /* Init current and alternate buffer */
+    term->current_buffer = term->normal_buffer = guac_terminal_buffer_alloc(initial_scrollback, &default_char);
+    term->alternate_buffer = guac_terminal_buffer_alloc(term->term_height, &default_char);
+
     /* Open STDIN pipe */
     if (pipe(term->stdin_pipe_fd)) {
         guac_error = GUAC_STATUS_SEE_ERRNO;
@@ -1358,6 +1358,9 @@ int guac_terminal_resize(guac_terminal* terminal, int width, int height) {
         terminal->scroll_end = rows - 1;
     }
 
+    /* Realloc alternate_buffer to fit terminal height */
+    terminal->alternate_buffer = guac_terminal_buffer_realloc(terminal->alternate_buffer, terminal->term_height);
+
     /* Notify scrollbar of resize */
     guac_terminal_scrollbar_parent_resized(terminal->scrollbar, 
         terminal->outer_width, terminal->outer_height, terminal->term_height);

src/terminal/terminal/buffer.h

@@ -49,6 +49,20 @@ typedef struct guac_terminal_buffer guac_terminal_buffer;
 guac_terminal_buffer* guac_terminal_buffer_alloc(int rows,
         const guac_terminal_char* default_character);
 
+/**
+ * Reallocate existing buffer to add or remove rows.
+ *
+ * @param buffer
+ *      The buffer to reallocate.
+ *
+ * @param rows
+ *      Number of scrollback rows
+ *
+ * @return
+ *      The reallocated buffer.
+ */
+guac_terminal_buffer* guac_terminal_buffer_realloc(guac_terminal_buffer* buffer, int rows);
+
 /**
  * Frees the given buffer.
  */