resize: don’t reflow text on alt screen

Alt screen applications normally reflow/readjust themselves on a
window resize.

When we do it too, the result is graphical glitches/flashes since our
re-flowed text is rendered in one frame, and the application re-flowed
text soon thereafter.

We can’t avoid rendering some kind of re-flowed frame, since we don’t
know when, or even if, the application will update itself. To avoid
glitches, we need to render, as closely as possible, what the
application itself will render shortly.

This is actually pretty simple; we just need to copy the visible
content over from the old grid to the new grid. We don’t bother with
text re-flow, but simply truncate long lines.

To simplify things, we simply cancel any active selection (since often
times, it will be corrupted anyway when the application redraws
itself).

Since we’re not reflowing text, there’s no need to translate e.g. the
cursor position - we just keep the current position (but bounded to
the new dimensions).

Fun thing: ‘less’ gets corrupted if we don’t leave the cursor at
the (new) bottom row. To handle this, we check if the cursor (before
resize) is at the bottom row, and if so, we move it to the new bottom
row.

Closes #221

CHANGELOG.md

@@ -92,6 +92,9 @@ means foot can be PGO:d in e.g. sandboxed build scripts. See
   this.
 * Sixel default maximum size is now 10000x10000 instead of the current
   window size.
+* Graphical glitches/flashes when resizing the window while running a
+  fullscreen application, i.e. the 'alt' screen
+  (https://codeberg.org/dnkl/foot/issues/221).
 
 
 ### Deprecated

grid.c

@@ -55,12 +55,125 @@ grid_row_free(struct row *row)
 }
 
 void
-grid_reflow(struct grid *grid, int new_rows, int new_cols,
-            int old_screen_rows, int new_screen_rows,
-            size_t tracking_points_count,
-            struct coord *const _tracking_points[static tracking_points_count],
-            size_t compose_count, const struct
-            composed composed[static compose_count])
+grid_resize_without_reflow(
+    struct grid *grid, int new_rows, int new_cols,
+    int old_screen_rows, int new_screen_rows)
+{
+    struct row *const *old_grid = grid->rows;
+    const int old_rows = grid->num_rows;
+    const int old_cols = grid->num_cols;
+
+    struct row **new_grid = xcalloc(new_rows, sizeof(new_grid[0]));
+
+    tll(struct sixel) untranslated_sixels = tll_init();
+    tll_foreach(grid->sixel_images, it)
+        tll_push_back(untranslated_sixels, it->item);
+    tll_free(grid->sixel_images);
+
+    int new_offset = 0;
+
+    /* Copy old lines, truncating them if old rows were longer */
+    for (int r = 0; r < min(old_screen_rows, new_screen_rows); r++) {
+        const int old_row_idx = (grid->offset + r) & (old_rows - 1);
+        const int new_row_idx = (new_offset + r) & (new_rows - 1);
+
+        const struct row *old_row = old_grid[old_row_idx];
+        assert(old_row != NULL);
+
+        struct row *new_row = grid_row_alloc(new_cols, false);
+        new_grid[new_row_idx] = new_row;
+
+        memcpy(new_row->cells,
+               old_row->cells,
+               sizeof(struct cell) * min(old_cols, new_cols));
+
+        new_row->dirty = old_row->dirty;
+        new_row->linebreak = false;
+
+        /* Clear "new" columns */
+        if (new_cols > old_cols) {
+            memset(&new_row->cells[old_cols], 0,
+                   sizeof(struct cell) * (new_cols - old_cols));
+            new_row->dirty = true;
+        }
+
+        /* Map sixels on current "old" row to current "new row" */
+        tll_foreach(untranslated_sixels, it) {
+            if (it->item.pos.row != old_row_idx)
+                continue;
+
+            struct sixel sixel = it->item;
+            sixel.pos.row = new_row_idx;
+
+            if (sixel.pos.col < new_cols)
+                tll_push_back(grid->sixel_images, sixel);
+            else
+                sixel_destroy(&it->item);
+            tll_remove(untranslated_sixels, it);
+        }
+    }
+
+    /* Clear "new" lines */
+    for (int r = min(old_screen_rows, new_screen_rows); r < new_screen_rows; r++) {
+        struct row *new_row = grid_row_alloc(new_cols, false);
+        new_grid[(new_offset + r) & (new_rows - 1)] = new_row;
+
+        memset(new_row->cells, 0, sizeof(struct cell) * new_cols);
+        new_row->linebreak = false;
+        new_row->dirty = true;
+    }
+
+    /* Free old grid */
+    for (int r = 0; r < grid->num_rows; r++)
+        grid_row_free(old_grid[r]);
+    free(grid->rows);
+
+    grid->rows = new_grid;
+    grid->num_rows = new_rows;
+    grid->num_cols = new_cols;
+
+    grid->view = grid->offset = new_offset;
+
+    /* Keep cursor at current position, but clamp to new dimensions */
+    struct coord cursor = grid->cursor.point;
+    if (cursor.row == old_screen_rows - 1) {
+        /* 'less' breaks if the cursor isn't at the bottom */
+        cursor.row = new_screen_rows - 1;
+    }
+    cursor.row = min(cursor.row, new_screen_rows - 1);
+    cursor.col = min(cursor.col, new_cols - 1);
+    grid->cursor.point = cursor;
+
+    struct coord saved_cursor = grid->saved_cursor.point;
+    if (saved_cursor.row == old_screen_rows - 1)
+        saved_cursor.row = new_screen_rows - 1;
+    saved_cursor.row = min(saved_cursor.row, new_screen_rows - 1);
+    saved_cursor.col = min(saved_cursor.col, new_cols - 1);
+    grid->saved_cursor.point = saved_cursor;
+
+    grid->cur_row = new_grid[(grid->offset + cursor.row) & (new_rows - 1)];
+    grid->cursor.lcf = false;
+    grid->saved_cursor.lcf = false;
+
+    /* Free sixels we failed to "map" to the new grid */
+    tll_foreach(untranslated_sixels, it)
+        sixel_destroy(&it->item);
+    tll_free(untranslated_sixels);
+
+#if defined(_DEBUG)
+    for (int r = 0; r < new_screen_rows; r++)
+        grid_row_in_view(grid, r);
+#endif
+}
+
+void
+grid_resize_and_reflow(
+    struct grid *grid, int new_rows, int new_cols,
+    int old_screen_rows, int new_screen_rows,
+    size_t tracking_points_count,
+    struct coord *const _tracking_points[static tracking_points_count],
+    size_t compose_count, const struct
+    composed composed[static compose_count])
 {
     struct row *const *old_grid = grid->rows;
     const int old_rows = grid->num_rows;
@@ -305,7 +418,6 @@ grid_reflow(struct grid *grid, int new_rows, int new_cols,
         grid_row_free(old_grid[r]);
     free(grid->rows);
 
-    grid->cur_row = new_grid[cursor.row];
     grid->rows = new_grid;
     grid->num_rows = new_rows;
     grid->num_cols = new_cols;
@@ -315,14 +427,15 @@ grid_reflow(struct grid *grid, int new_rows, int new_cols,
     while (cursor.row < 0)
         cursor.row += grid->num_rows;
     cursor.row = min(cursor.row, new_screen_rows - 1);
-    assert(cursor.col >= 0 && cursor.col < new_cols);
+    cursor.col = min(cursor.col, new_cols - 1);
 
     saved_cursor.row -= grid->offset;
     while (saved_cursor.row < 0)
         saved_cursor.row += grid->num_rows;
     saved_cursor.row = min(saved_cursor.row, new_screen_rows - 1);
-    assert(saved_cursor.col >= 0 && saved_cursor.col < new_cols);
+    saved_cursor.col = min(saved_cursor.col, new_cols - 1);
 
+    grid->cur_row = new_grid[(grid->offset + cursor.row) & (new_rows - 1)];
     grid->cursor.point = cursor;
     grid->saved_cursor.point = saved_cursor;
 

grid.h

@@ -6,11 +6,16 @@
 void grid_swap_row(struct grid *grid, int row_a, int row_b);
 struct row *grid_row_alloc(int cols, bool initialize);
 void grid_row_free(struct row *row);
-void grid_reflow(
+
+void grid_resize_without_reflow(
+    struct grid *grid, int new_rows, int new_cols,
+    int old_screen_rows, int new_screen_rows);
+
+void grid_resize_and_reflow(
     struct grid *grid, int new_rows, int new_cols,
     int old_screen_rows, int new_screen_rows,
     size_t tracking_points_count,
-    struct coord *const tracking_points[static tracking_points_count],
+    struct coord *const _tracking_points[static tracking_points_count],
     size_t compose_count,
     const struct composed composed[static compose_count]);
 

render.c

@@ -2276,23 +2276,22 @@ maybe_resize(struct terminal *term, int width, int height, bool force)
         goto damage_view;
     }
 
+    if (term->grid == &term->alt)
+        selection_cancel(term);
+
     struct coord *const tracking_points[] = {
         &term->selection.start,
         &term->selection.end,
     };
 
-    /* Reflow grids */
-    grid_reflow(
+    /* Resize grids */
+    grid_resize_and_reflow(
         &term->normal, new_normal_grid_rows, new_cols, old_rows, new_rows,
-        term->grid == &term->normal ? ALEN(tracking_points) : 0,
-        term->grid == &term->normal ? tracking_points : NULL,
+        ALEN(tracking_points), tracking_points,
         term->composed_count, term->composed);
 
-    grid_reflow(
-        &term->alt, new_alt_grid_rows, new_cols, old_rows, new_rows,
-        term->grid == &term->alt ? ALEN(tracking_points) : 0,
-        term->grid == &term->alt ? tracking_points : NULL,
-        term->composed_count, term->composed);
+    grid_resize_without_reflow(
+        &term->alt, new_alt_grid_rows, new_cols, old_rows, new_rows);
 
     /* Reset tab stops */
     tll_free(term->tab_stops);