Some minor cleanup of UI code

- Make gui_window_end returns the size of the window, so that we can use
that for manual layout.

- Clamp the size of the windows to the screen size, so that we can
  scroll them.

src/gui.cpp

@@ -748,37 +748,51 @@ void gui_window_begin(const char *label, float x, float y, float w, float h,
         ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize |
         ImGuiWindowFlags_NoDecoration;
     float max_h;
+    ImGuiStorage *storage = ImGui::GetStateStorage();
+    ImGuiID key;
+    float *last_y;
 
+    ImGui::PushID(label);
     if (!gui->can_move_window)
         flags |= ImGuiWindowFlags_NoMove;
     if (gui->scrolling)
         flags |= ImGuiWindowFlags_NoMouseInputs;
     ImGui::SetNextWindowPos(ImVec2(x, y),
             moved == NULL ? ImGuiCond_Always : ImGuiCond_Appearing);
     ImGui::SetNextWindowSize(ImVec2(w, h));
+
+    key = ImGui::GetID("lasty");
+    last_y = storage->GetFloatRef(key, y);
+
     if (h == 0) {
-        max_h = ImGui::GetMainViewport()->Size.y - y;
+        max_h = ImGui::GetMainViewport()->Size.y - *last_y;
         ImGui::SetNextWindowSizeConstraints(
                 ImVec2(0, 0), ImVec2(FLT_MAX, max_h));
     }
     ImGui::Begin(label, NULL, flags);
 
     if (moved != NULL) {
         *moved = ImGui::GetWindowPos() != ImVec2(x, y);
+        *last_y = ImGui::GetWindowPos().y;
     }
 
     ImGui::BeginGroup();
 }
 
-void gui_window_end(void)
+gui_window_ret_t gui_window_end(void)
 {
-
+    gui_window_ret_t ret = {};
     ImGui::EndGroup();
-    if (!GUI_HAS_SCROLLBARS) {
+    if (!GUI_HAS_SCROLLBARS && !gui->can_move_window) {
         if (gui_pan_scroll_behavior())
             gui->scrolling |= 1;
     }
+    ret.h = ImGui::GetWindowHeight();
+    ret.w = ImGui::GetWindowWidth();
     ImGui::End();
+    ImGui::PopID();
+
+    return ret;
 }
 
 bool gui_input_int(const char *label, int *v, int minv, int maxv)

src/gui.h

@@ -35,10 +35,15 @@
 #   define GUI_PANEL_WIDTH_LARGE 400
 #endif
 
+typedef struct {
+    float h;
+    float w;
+} gui_window_ret_t;
+
 void gui_window_begin(const char *label, float x, float y, float w, float h,
                       bool *moved);
 
-void gui_window_end(void);
+gui_window_ret_t gui_window_end(void);
 
 bool gui_want_capture_mouse(void);
 

src/gui/app.c

@@ -138,9 +138,8 @@ void gui_app(void)
 
     gui_window_begin("Top Bar", x, y, 0, 0, NULL);
     gui_top_bar();
-    gui_window_end();
+    y += gui_window_end().h;
 
-    y += ICON_HEIGHT + 28;
     gui_window_begin("Left Bar", x, y, 0, 0, NULL);
     render_left_panel();
     gui_window_end();