terminal: break out scaling factor updating, and reduce number of calls to render_resize()

Break out the logic that updates the terminal’s scaling factor value,
from render_resize(), to a new function, term_update_scale(). This
allows us to update the scaling factor without a full grid resize.

We also change how we pick the scaling factor (when fractional scaling
is not in use). Before, we’d use the highest scaling factor from all
monitors we were mapped on. Now, we use the scaling factor from the
monitor we were *last* mapped on.

Then, add a boolean parameter to term_set_fonts(), and when
false, *don’t* call render_resize_force().

Also change term_font_dpi_changed() to only return true if the font
was changed in any way.

Finally, rewrite update_term_for_output_change() to:

* Call term_update_scale() before doing anything else
* Call render_resize{,_force} *last*, and *only* if either the scale
  or the fonts were updated.

This fixes several things:

* A bug where we failed to update the fonts when fractional scaling
  was in use, and we guessed the initial scale/DPI wrong. The bug
  happened because updated the internal "preferred" scale value, and a
  later call to render_resize() updated the terminal’s scale value,
  but since that code path didn’t call term_font_dpi_changed() (and it
  shouldn’t), the fonts weren’t resized properly.

* It ensures we only resize the grid *once* when the scaling factor,
  or DPI is changed. Before this, we’d resize it twice. And this
  happened when e.g. dragging the window between monitors.

render.c

@@ -3841,21 +3841,7 @@ maybe_resize(struct terminal *term, int width, int height, bool force)
     if (term->cell_width == 0 && term->cell_height == 0)
         return false;
 
-    float scale = -1;
-    if (wayl_fractional_scaling(term->wl)) {
-        scale = term->window->scale;
-    } else {
-        tll_foreach(term->window->on_outputs, it) {
-            if (it->item->scale > scale)
-                scale = it->item->scale;
-        }
-    }
-
-    if (scale < 0.) {
-        /* Haven't 'entered' an output yet? */
-        scale = term->scale;
-    }
-
+    const float scale = term->scale;
     width = round(width * scale);
     height = round(height * scale);
 
@@ -3942,9 +3928,9 @@ maybe_resize(struct terminal *term, int width, int height, bool force)
     /* Drop out of URL mode */
     urls_reset(term);
 
+    LOG_DBG("resized: size=%dx%d (scale=%.2f)", width, height, term->scale);
     term->width = width;
     term->height = height;
-    term->scale = scale;
 
     const uint32_t scrollback_lines = term->render.scrollback_lines;
 
@@ -4148,12 +4134,11 @@ maybe_resize(struct terminal *term, int width, int height, bool force)
 
     sixel_reflow(term);
 
-#if defined(_DEBUG) && LOG_ENABLE_DBG
-    LOG_DBG("resize: %dx%d, grid: cols=%d, rows=%d "
+    LOG_DBG("resized: grid: cols=%d, rows=%d "
             "(left-margin=%d, right-margin=%d, top-margin=%d, bottom-margin=%d)",
-            term->width, term->height, term->cols, term->rows,
-            term->margins.left, term->margins.right, term->margins.top, term->margins.bottom);
-#endif
+            term->cols, term->rows,
+            term->margins.left, term->margins.right,
+            term->margins.top, term->margins.bottom);
 
     if (term->scroll_region.start >= term->rows)
         term->scroll_region.start = 0;
@@ -4295,7 +4280,7 @@ render_xcursor_update(struct seat *seat)
 #endif
 
     LOG_DBG("setting %scursor shape using a client-side cursor surface",
-            shape == CURSOR_SHAPE_CUSTOM ? "custom " : "");
+            seat->pointer.shape == CURSOR_SHAPE_CUSTOM ? "custom " : "");
 
     const float scale = seat->pointer.scale;
     struct wl_cursor_image *image = seat->pointer.cursor->images[0];

terminal.c

@@ -733,7 +733,8 @@ term_line_height_update(struct terminal *term)
 }
 
 static bool
-term_set_fonts(struct terminal *term, struct fcft_font *fonts[static 4])
+term_set_fonts(struct terminal *term, struct fcft_font *fonts[static 4],
+               bool resize_grid)
 {
     for (size_t i = 0; i < 4; i++) {
         xassert(fonts[i] != NULL);
@@ -777,11 +778,15 @@ term_set_fonts(struct terminal *term, struct fcft_font *fonts[static 4])
 
     sixel_cell_size_changed(term);
 
-    /* Use force, since cell-width/height may have changed */
-    render_resize_force(
-        term,
-        round(term->width / term->scale),
-        round(term->height / term->scale));
+    /* Optimization - some code paths (are forced to) call
+     * render_resize() after this function */
+    if (resize_grid) {
+        /* Use force, since cell-width/height may have changed */
+        render_resize_force(
+            term,
+            round(term->width / term->scale),
+            round(term->height / term->scale));
+    }
     return true;
 }
 
@@ -899,7 +904,7 @@ font_loader_thread(void *_data)
 }
 
 static bool
-reload_fonts(struct terminal *term)
+reload_fonts(struct terminal *term, bool resize_grid)
 {
     const struct config *conf = term->conf;
 
@@ -1026,7 +1031,7 @@ reload_fonts(struct terminal *term)
         }
     }
 
-    return success ? term_set_fonts(term, fonts) : success;
+    return success ? term_set_fonts(term, fonts, resize_grid) : success;
 }
 
 static bool
@@ -1044,7 +1049,7 @@ load_fonts_from_conf(struct terminal *term)
         }
     }
 
-    return reload_fonts(term);
+    return reload_fonts(term, true);
 }
 
 static void fdm_client_terminated(
@@ -1987,7 +1992,7 @@ term_font_size_adjust_by_points(struct terminal *term, float amount)
         }
     }
 
-    return reload_fonts(term);
+    return reload_fonts(term, true);
 }
 
 static bool
@@ -2010,7 +2015,7 @@ term_font_size_adjust_by_pixels(struct terminal *term, int amount)
         }
     }
 
-    return reload_fonts(term);
+    return reload_fonts(term, true);
 }
 
 static bool
@@ -2034,7 +2039,7 @@ term_font_size_adjust_by_percent(struct terminal *term, bool increment, float pe
         }
     }
 
-    return reload_fonts(term);
+    return reload_fonts(term, true);
 }
 
 bool
@@ -2071,6 +2076,36 @@ term_font_size_reset(struct terminal *term)
     return load_fonts_from_conf(term);
 }
 
+bool
+term_update_scale(struct terminal *term)
+{
+    const struct wl_window *win = term->window;
+
+    /*
+     * We have a number of “sources” we can use as scale. We choose
+     * the scale in the following order:
+     *
+     *  - “preferred” scale, from the fractional-scale-v1 protocol
+     *  - scaling factor of output we most recently were mapped on
+     *  - if we’re not mapped, use the scaling factor from the first
+     *    available output.
+     *  - if there aren’t any outputs available, use 1.0
+     */
+    const float new_scale =
+        (wayl_fractional_scaling(term->wl) && win->scale > 0.
+         ? win->scale
+         : (tll_length(win->on_outputs) > 0
+            ? tll_back(win->on_outputs)->scale
+            : 1.));
+
+    if (new_scale == term->scale)
+        return false;
+
+    LOG_DBG("scaling factor changed: %.2f -> %.2f", term->scale, new_scale);
+    term->scale = new_scale;
+    return true;
+}
+
 bool
 term_font_dpi_changed(struct terminal *term, float old_scale)
 {
@@ -2099,9 +2134,9 @@ term_font_dpi_changed(struct terminal *term, float old_scale)
     term->font_is_sized_by_dpi = will_scale_using_dpi;
 
     if (!need_font_reload)
-        return true;
+        return false;
 
-    return reload_fonts(term);
+    return reload_fonts(term, false);
 }
 
 void
@@ -3500,7 +3535,7 @@ term_update_ascii_printer(struct terminal *term)
 
 #if defined(_DEBUG) && LOG_ENABLE_DBG
     if (term->ascii_printer != new_printer) {
-        LOG_DBG("§switching ASCII printer %s -> %s",
+        LOG_DBG("switching ASCII printer %s -> %s",
                 term->ascii_printer == &ascii_printer_fast ? "fast" : "generic",
                 new_printer == &ascii_printer_fast ? "fast" : "generic");
     }

terminal.h

@@ -736,6 +736,7 @@ bool term_to_slave(struct terminal *term, const void *data, size_t len);
 bool term_paste_data_to_slave(
     struct terminal *term, const void *data, size_t len);
 
+bool term_update_scale(struct terminal *term);
 bool term_font_size_increase(struct terminal *term);
 bool term_font_size_decrease(struct terminal *term);
 bool term_font_size_reset(struct terminal *term);

wayland.c

@@ -396,17 +396,35 @@ static const struct wl_seat_listener seat_listener = {
 static void
 update_term_for_output_change(struct terminal *term)
 {
-    if (tll_length(term->window->on_outputs) == 0)
-        return;
+    const float old_scale = term->scale;
+    const float logical_width = term->width / term->scale;
+    const float logical_height = term->height / term->scale;
 
-    float old_scale = term->scale;
-
-    render_resize(term,
-                  round(term->width / term->scale),
-                  round(term->height / term->scale));
-    term_font_dpi_changed(term, old_scale);
+    /* Note: order matters! term_update_scale() must come first */
+    bool scale_updated = term_update_scale(term);
+    bool fonts_updated = term_font_dpi_changed(term, old_scale);
     term_font_subpixel_changed(term);
+
     csd_reload_font(term->window, old_scale);
+
+    if (fonts_updated) {
+        /*
+         * If the fonts have been updated, the cell dimensions have
+         * changed. This requires a “forced” resize, since the surface
+         * buffer dimensions may not have been updated (in which case
+         * render_size() normally shortcuts and returns early).
+         */
+        render_resize_force(term, round(logical_width), round(logical_height));
+    }
+
+    else if (scale_updated) {
+        /*
+         * A scale update means the surface buffer dimensions have
+         * been updated, even though the window logical dimensions
+         * haven’t changed.
+         */
+        render_resize(term, round(logical_width), round(logical_height));
+    }
 }
 
 static void