Keep blocks still when switching layers

- The main block reserves the width and rows of its biggest layer, so the
  numpad and function row no longer shift between letters and symbols
- Smaller layers stretch into the room: stretch keys widen, rows grow
  taller (rows above the layer, like the function row, keep their height)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/layout/LayoutModel.cpp

@@ -1,5 +1,6 @@
 #include "LayoutModel.h"
 
+#include <algorithm>
 #include <limits>
 
 namespace {
@@ -183,8 +184,16 @@ QList<Block> LayoutModel::composeBlocks() const
         if (section.placement == Section::Placement::Above && shown(section.id))
             main.rows += section.rows;
     }
-    if (layer && shown(MainSectionId))
+    if (layer && shown(MainSectionId)) {
+        // Room for the biggest layer, so switching layers keeps the other blocks still.
+        main.fixedRows = main.rows.size();
+        for (const Layer &other : m_layout.layers) {
+            main.minRows = std::max(main.minRows, main.fixedRows + other.rows.size());
+            for (const Row &row : other.rows)
+                main.minUnits = std::max(main.minUnits, rowUnits(row));
+        }
         main.rows += layer->rows;
+    }
 
     QList<Block> blocks;
     if (!main.rows.isEmpty())