Addresses issue #879

src/Fl_Terminal.cxx

@@ -1210,15 +1210,48 @@ void Fl_Terminal::update_scrollbar(void) {
   scrollbar->redraw();     // redraw scroll always
 }
 
-// Refit the display to match screen
+// Refit the display - (display_rows()/cols()) to match screen (scrn_.h()/w())
+//   This implements an xterm-like resizing behavior. Refer to README-Fl_Terminal.txt,
+//   section "TERMINAL RESIZING" for a diagram of cases showing how this is implemented.
+//   See also issue #879.
+//
 void Fl_Terminal::refit_disp_to_screen(void) {
-  int dh = h_to_row(scrn_.h());                    // height in rows for tty pixel height
+  int dh         = h_to_row(scrn_.h());         // disp height: in rows for tty pixel height
-  int dw = MAX(w_to_col(scrn_.w()), disp_cols());  // width in cols for tty pixel width - enlarge only!
+  int dw         = MAX(w_to_col(scrn_.w()), disp_cols()); // disp width: in cols from pixel width - enlarge only!
-  int drows     = clamp(dh, 2,  dh);               // 2 rows minimum
+  int drows      = clamp(dh, 2,  dh);           // disp rows: 2 rows minimum
-  int dcols     = clamp(dw, 10, dw);               // 10 cols minimum
+  int dcols      = clamp(dw, 10, dw);           // disp cols: 10 cols minimum
-  int drow_diff = drows - display_rows();          // change in rows?
+  int drow_diff  = drows - display_rows();      // disp row diff: change in rows?
-  ring_.resize(drows, dcols, hist_rows(), *current_style_);
+  int is_enlarge = drows >= display_rows();     // enlarging display size?
-  cursor_.scroll(-drow_diff);
+
+  // NOTE: Zeroing scrollbar can be avoided if we took the scroll position
+  //       into account for the below calculations. But for now..
+  scrollbar->value(0);                          // force scrollbar to bottom before refit
+
+  if (drow_diff) {                              // change in display rows means shrink|enlarge
+    if (is_enlarge) {                           // enlarging widget?
+      for (int i=0; i<drow_diff; i++) {         // carefully loop thru each change
+        if (history_use() > 0) {                // CASE 1: Drag lines down from history
+          cursor_.scroll(-1);                   // cursor chases ring_.resize()
+        } else {                                // CASE 2: add blank lines below cursor
+          scroll(1);                            // scroll up to create blank lines at bottom
+        }
+        // Handle enlarging ring's display
+        ring_.resize(display_rows()+1, dcols, hist_rows(), *current_style_);
+      }
+    } else {                                    // shrinking widget?
+      for (int i=0; i<(-drow_diff); i++) {      // carefully loop thru each row change
+        int cur_row   = cursor_.row();          // cursor row
+        int below_cur = (drows > cur_row);      // shrink is below cursor row?
+        if (below_cur) {                        // CASE 3: shrinking below cursor? drop lines below
+          ring_.disp_rows(display_rows() - 1);  // effectively "deletes" lines below cursor
+        } else {                                // CASE 4: need to move cursor + lines up into hist
+          cursor_up(-1, false);                 // move cursor down to follow ring_.resize()
+          // Handle shrinking ring's display up into history
+          ring_.resize(display_rows()-1, dcols, hist_rows(), *current_style_);
+        }
+      }
+    }
+  }
   clear_mouse_selection();
   update_screen(false);
 }

src/README-Fl_Terminal.txt

@@ -303,7 +303,51 @@ Some terminal operations necessarily call redraw() directly, such as interactive
 selection, or during user scrolling the terminal's scrollbar, where it's important there's
 no delay in what the user sees while interacting directly with the widget.
 
+TERMINAL RESIZING
+=================
+Resizing currently follows xterm(1) style behavior:
 
+    CASE 1: ENLARGE, USING HISTORY:
+           -2  Line 1   ┐_ scrollback
+           -1  Line 2   ┘  history
+             ┌──────────────────┐              ┌──────────────────┐
+            1│ Hello!           │             1│ Line 1           │
+            2│ ▒                │   enlarge   2│ Line 2           │
+             └──────────────────┘ ──┐         3│ Hello!           │
+                                    │         4│ ▒                │
+                                    └──────>   └──────────────────┘
+    ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄
+    CASE 2: ENLARGE, ADDING LINES BELOW CURSOR
+             ┌──────────────────┐              ┌──────────────────┐
+            1│ Line 1           │             1│ Line 1           │
+            2│ Line 2           │   enlarge   2│ Line 2           │
+            3│ Hello!           │             3│ Hello!           │
+            4│ ▒                │             4│ ▒                │
+             └──────────────────┘ ──┐         5│                  │
+                                    │         6│                  │
+                                    └──────>   └──────────────────┘
+    ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄
+    CASE 3: SHRINK, TRIMMING LINES BELOW CURSOR
+             ┌──────────────────┐            ┌──────────────────┐
+            1│ Line 1           │           1│ Line 1           │
+            2│ Line 2           │           2│ Line 2           │
+            3│ Hello!           │   resize  3│ Hello!           │
+            4│ ▒                │           4│ ▒                │
+            5│                  │   ╭────>   └──────────────────┘
+            6│                  │   │       5    ┐ Lines below cursor erased
+             └──────────────────┘ ──╯       6    ┘ (regardless of contents)
+    ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄ ┄
+    CASE 4: SHRINK, PUSH TO HISTORY:       -2  Line 1   ┐_ moved into
+                                           -1  Line 2   ┘  scrollback history
+             ┌──────────────────┐            ┌──────────────────┐
+            1│ Line 1           │           1│ Hello!           │
+            2│ Line 2           │   resize  2│ ▒                │
+            3│ Hello!           │   ╭────>   └──────────────────┘
+            4│ ▒                │   │
+             └──────────────────┘ ──╯
+
+These case numbers (CASE 1 tru 4) are referenced in the source code for
+Fl_Terminal::refit_disp_to_screen(void).
 
 
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