term: stash last known DPI, and use after a unmapped/mapped sequence

A compositor may unmap, and then remap the window, for example when
the window is minimized, or if the user switches workspace.

With DPI aware rendering, we *need* to know on which output we're
mapped, in order to use the correct DPI. This means the first frame we
render, before being mapped, always guesses the DPI.

In an unmap/map sequence, guessing the wrong DPI means the window will
flicker.

Fix by stashing the last used DPI value, and use that instead of
guessing.

This means the *only* time we _actually_ guess the DPI, is the very
first frame, when starting up foot.

CHANGELOG.md

@@ -68,6 +68,9 @@
 * Crash when printing double-width (or longer) characters to, or near,
   the last column, when auto-wrap (private mode 7) has been disabled.
 * Dynamically sized sixel being trimmed to nothing.
+* Flickering with `dpi-aware=yes` and window is unmapped/remapped
+  (some compositors do this when window is minimized), in a
+  multi-monitor setup with different monitor DPIs.
 
 
 ### Security

terminal.c

@@ -858,9 +858,25 @@ get_font_dpi(const struct terminal *term)
     xassert(tll_length(term->wl->monitors) > 0);
 
     const struct wl_window *win = term->window;
-    const struct monitor *mon = tll_length(win->on_outputs) > 0
-        ? tll_back(win->on_outputs)
-        : &tll_front(term->wl->monitors);
+    const struct monitor *mon = NULL;
+
+    if (tll_length(win->on_outputs) > 0)
+        mon = tll_back(win->on_outputs);
+    else {
+        if (term->font_dpi_before_unmap > 0.) {
+            /*
+             * Use last known "good" DPI
+             *
+             * This avoids flickering when window is unmapped/mapped
+             * (some compositors do this when a window is minimized),
+             * on a multi-monitor setup with different monitor DPIs.
+             */
+            return term->font_dpi_before_unmap;
+        }
+
+        if (tll_length(term->wl->monitors) > 0)
+            mon = &tll_front(term->wl->monitors);
+    }
 
     if (term_fractional_scaling(term))
         return mon != NULL ? mon->dpi.physical : 96.;
@@ -1182,6 +1198,7 @@ term_init(const struct config *conf, struct fdm *fdm, struct reaper *reaper,
             xmalloc(sizeof(term->font_sizes[3][0]) * conf->fonts[3].count),
         },
         .font_dpi = 0.,
+        .font_dpi_before_unmap = -1.,
         .font_subpixel = (conf->colors.alpha == 0xffff  /* Can't do subpixel rendering on transparent background */
                           ? FCFT_SUBPIXEL_DEFAULT
                           : FCFT_SUBPIXEL_NONE),
@@ -2250,6 +2267,7 @@ term_font_dpi_changed(struct terminal *term, float old_scale)
     }
 
     term->font_dpi = dpi;
+    term->font_dpi_before_unmap = dpi;
     term->font_is_sized_by_dpi = will_scale_using_dpi;
 
     if (!need_font_reload)

terminal.h

@@ -406,6 +406,7 @@ struct terminal {
     struct config_font *font_sizes[4];
     struct pt_or_px font_line_height;
     float font_dpi;
+    float font_dpi_before_unmap;
     bool font_is_sized_by_dpi;
     int16_t font_x_ofs;
     int16_t font_y_ofs;