In ltop, show >=2 rows in MDT window if displayed

Make MDT window always show at least 2 MDTs if there is room to have an
MDT window at all.

Clean up window sizing logic to make it easier to reason about.

Fix a bug in logic to proportionately allocate rows to OST and MDT
windows.  MDT window detail window only had 1 row if the window was too
small to show all targets.

utils/ltop.c

@@ -305,30 +305,30 @@ void
 size_windows(int total_size, int mdts, int osts,
              int *mdtlines, int *ostlines)
 {
-    int headers = 2;    /* max lines occupied by MDT and OST headers */
-    int extra = total_size - (TOPWIN_LINES + mdts + osts + headers);
-
-    if (total_size <= TOPWIN_LINES) {
+    if (total_size <= (TOPWIN_LINES+2)) {
         *mdtlines = 0;
         *ostlines = 0;
     } else {
-        if (mdts < 2 || (total_size < (TOPWIN_LINES + headers + 2))) {
+        if (mdts < 2) {
             // no need for per-mdt lines OR
-            // only big enough for topwin + small OST section
             *mdtlines = 0;
-            *ostlines = total_size - TOPWIN_LINES;
         } else {
-            /* big enough for both MDT and OST sections, and >1 MDT */
-            if (extra >= 0) { 
-                /* enough room to display everything */
-                *mdtlines = mdts+1;
-                *ostlines = osts+1;
+            if (total_size <= (TOPWIN_LINES + 5)) {
+                // only big enough for topwin + small OST section
+                *mdtlines = 0;
             } else {
-                /* OST and MDT section sizes proportional to target counts */
-                *mdtlines = mdts + 1 + ((float)extra*mdts) / ((float)mdts+osts);
-                *ostlines = total_size - (TOPWIN_LINES + *mdtlines);
+                int detail_size = total_size - TOPWIN_LINES;
+                /*
+                 * OST and MDT section sizes proportional to target counts,
+                 * except that we allow space for at least 2 MDTs since we know
+                 * there is more than one MDT.
+                 */
+                *mdtlines = 2 + ((float)detail_size*mdts) / ((float)mdts+osts);
+                *mdtlines = (*mdtlines < 3 ? 3 : *mdtlines);
+                *mdtlines = (*mdtlines > (mdts+2) ? (mdts+2) : *mdtlines);
             }
         }
+        *ostlines = total_size - (TOPWIN_LINES + *mdtlines);
     }
 }