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/KeyGeometry.h

@@ -12,16 +12,23 @@ struct KeyboardMetrics {
 };
 
 // The keys on screen: blocks of rows, side by side.
+// A block can reserve room (minUnits, minRows) so that swapping its rows, e.g.
+// switching layers, moves nothing else; its keys stretch into the room.
 struct Block {
     QList<Row> rows;
-    bool fillHeight = false; // rows grow to span the full height instead of sharing the row height
+    bool fillHeight = false; // spans the full height instead of max(rows, minRows) shared rows
+    qreal minUnits = 0;      // reserved width
+    qsizetype minRows = 0;   // reserved height, in shared rows
+    qsizetype fixedRows = 0; // leading rows that keep the shared height when the block stretches
 };
 
 // Lays out blocks to fill `size`. Returns one rect per key, block by block in
 // row order (spacers included). One key unit has the same width everywhere;
 // rows share one height, set by the tallest block that doesn't fill, and
-// shorter blocks sit at the bottom. In each block, stretch keys grow to make
-// their row as wide as the widest one, and remaining shorter rows are centered.
+// shorter blocks sit at the bottom. A block given more height than its rows
+// need (fillHeight, minRows) grows its rows after fixedRows to take it up.
+// In each block, stretch keys grow to make their row as wide as the widest
+// one (or minUnits), and remaining shorter rows are centered.
 QList<QRectF> computeKeyRects(const QList<Block> &blocks, QSizeF size, KeyboardMetrics metrics = {});
 inline QList<QRectF> computeKeyRects(const Layer &layer, QSizeF size, KeyboardMetrics metrics = {})
 {
@@ -32,5 +39,5 @@ inline QList<QRectF> computeKeyRects(const Layer &layer, QSizeF size, KeyboardMe
 qreal rowUnits(const Row &row);
 // Width of all blocks in key units, ignoring gaps.
 qreal totalUnits(const QList<Block> &blocks);
-// Rows of the tallest block that sets the row height (see computeKeyRects).
+// Shared rows of the tallest block that sets the row height (see computeKeyRects).
 qsizetype totalRows(const QList<Block> &blocks);